chore(variants): drop the dead MAX_MQTT_BROKERS build flag

Nothing has ever read it. The slot count comes from two other places: the
persisted array size (MAX_MQTT_SLOTS / RUNTIME_MQTT_SLOTS in MQTTPresets.h) and
the runtime cap MQTTBridge::getMaxActiveSlots(), which answers 5 with PSRAM and
2 without. A flag reading MAX_MQTT_BROKERS=3 sitting in every observer env
implies a third, lower limit that does not exist — it cost me a wrong answer
about how many MQTT slots a Heltec V4 exposes.

36 lines across 15 variants, all of them =3. Removing an unread -D cannot change
code: heltec_v4_repeater_observer_mqtt builds to a byte-identical size before and
after (1647744). The image checksum does differ, but so does it between two
clean builds of untouched source — ESP-IDF stamps the app descriptor with the
build time — so size is the meaningful comparison here.

All 606 envs still parse; heltec_v4, heltec_v3, station_g2, rak3112 and
xiao_s3_wio observer targets all build.

variants/heltec_t190/platformio.ini

@@ -113,7 +113,6 @@ build_flags =
   -D ADMIN_PASSWORD='"password"'
   -D MAX_NEIGHBOURS=50
   -D WITH_MQTT_BRIDGE=1
-  -D MAX_MQTT_BROKERS=3
   -D MQTT_MAX_PACKET_SIZE=1024
   -D MQTT_DEBUG=1
   -D MESH_PACKET_LOGGING=1
@@ -206,7 +205,6 @@ build_flags =
   -D ROOM_PASSWORD='"hello"'
   -D WITH_MQTT_BRIDGE=1
   -D MAX_NEIGHBOURS=50
-  -D MAX_MQTT_BROKERS=3
   -D MQTT_MAX_PACKET_SIZE=1024
   -D MQTT_DEBUG=1
   -D MESH_PACKET_LOGGING=1

variants/heltec_tracker_v2/platformio.ini

@@ -187,7 +187,6 @@ build_flags =
   -D WITH_MQTT_BRIDGE=1
 ; Non-PSRAM board: neighbors table costs ~4 KB of internal DRAM (see MQTTBridge.h).
   -D MQTT_NEIGHBORS_WITHOUT_PSRAM=1
-  -D MAX_MQTT_BROKERS=3
   -D MQTT_MAX_PACKET_SIZE=1024
   -D MQTT_DEBUG=1
   -D CONFIG_MBEDTLS_CERTIFICATE_BUNDLE=y
@@ -225,7 +224,6 @@ build_flags =
   -D WITH_MQTT_BRIDGE=1
 ; Non-PSRAM board: neighbors table costs ~4 KB of internal DRAM (see MQTTBridge.h).
   -D MQTT_NEIGHBORS_WITHOUT_PSRAM=1
-  -D MAX_MQTT_BROKERS=3
   -D MQTT_MAX_PACKET_SIZE=1024
   -D MQTT_DEBUG=1
   -D CONFIG_MBEDTLS_CERTIFICATE_BUNDLE=y
@@ -264,7 +262,6 @@ build_flags =
 ; Non-PSRAM board: neighbors table costs ~4 KB of internal DRAM (see MQTTBridge.h).
   -D MQTT_NEIGHBORS_WITHOUT_PSRAM=1
   -D MAX_NEIGHBOURS=50
-  -D MAX_MQTT_BROKERS=3
   -D MQTT_MAX_PACKET_SIZE=1024
   -D MQTT_DEBUG=1
   -D CONFIG_MBEDTLS_CERTIFICATE_BUNDLE=y
@@ -303,7 +300,6 @@ build_flags =
 ; Non-PSRAM board: neighbors table costs ~4 KB of internal DRAM (see MQTTBridge.h).
   -D MQTT_NEIGHBORS_WITHOUT_PSRAM=1
   -D MAX_NEIGHBOURS=50
-  -D MAX_MQTT_BROKERS=3
   -D MQTT_MAX_PACKET_SIZE=1024
   -D MQTT_DEBUG=1
   -D CONFIG_MBEDTLS_CERTIFICATE_BUNDLE=y

variants/heltec_v3/platformio.ini

@@ -126,7 +126,6 @@ build_flags =
   -D WITH_MQTT_BRIDGE=1
 ; Non-PSRAM board: neighbors table costs ~4 KB of internal DRAM (see MQTTBridge.h).
   -D MQTT_NEIGHBORS_WITHOUT_PSRAM=1
-  -D MAX_MQTT_BROKERS=3
   -D MQTT_MAX_PACKET_SIZE=1024
   -D MQTT_DEBUG=1
 ; Periodic 30s heap/pub-stats serial log — enable only for debugging (use `get mqtt.stats` on demand instead).
@@ -212,7 +211,6 @@ build_flags =
 ; Non-PSRAM board: neighbors table costs ~4 KB of internal DRAM (see MQTTBridge.h).
   -D MQTT_NEIGHBORS_WITHOUT_PSRAM=1
   -D MAX_NEIGHBOURS=50
-  -D MAX_MQTT_BROKERS=3
   -D MQTT_MAX_PACKET_SIZE=1024
   -D MQTT_DEBUG=1
 ;  -D MESH_PACKET_LOGGING=1
@@ -509,7 +507,6 @@ build_flags =
   -D WITH_MQTT_BRIDGE=1
 ; Non-PSRAM board: neighbors table costs ~4 KB of internal DRAM (see MQTTBridge.h).
   -D MQTT_NEIGHBORS_WITHOUT_PSRAM=1
-  -D MAX_MQTT_BROKERS=3
   -D MQTT_MAX_PACKET_SIZE=1024
   -D MQTT_DEBUG=1
 ; Periodic 30s heap/pub-stats serial log — enable only for debugging (use `get mqtt.stats` on demand instead).
@@ -561,7 +558,6 @@ build_flags =
 ; Non-PSRAM board: neighbors table costs ~4 KB of internal DRAM (see MQTTBridge.h).
   -D MQTT_NEIGHBORS_WITHOUT_PSRAM=1
   -D MAX_NEIGHBOURS=50
-  -D MAX_MQTT_BROKERS=3
   -D MQTT_MAX_PACKET_SIZE=1024
   -D MQTT_DEBUG=1
 ; Keep default observer profile less verbose to reduce runtime contention.

variants/heltec_v4/platformio.ini

@@ -162,7 +162,6 @@ build_flags =
   -D ADMIN_PASSWORD='"password"'
   -D MAX_NEIGHBOURS=50
   -D WITH_MQTT_BRIDGE=1
-  -D MAX_MQTT_BROKERS=3
   -D MQTT_MAX_PACKET_SIZE=1024
   -D MQTT_DEBUG=1
 ;  -D MQTT_MEMORY_DEBUG=1
@@ -213,7 +212,6 @@ build_flags =
   -D ADMIN_PASSWORD='"password"'
   -D MAX_NEIGHBOURS=50
   -D WITH_MQTT_BRIDGE=1
-  -D MAX_MQTT_BROKERS=3
   -D MQTT_MAX_PACKET_SIZE=1024
   -D MQTT_DEBUG=1
 ;  -D MQTT_MEMORY_DEBUG=1
@@ -316,7 +314,6 @@ build_flags =
   -D ROOM_PASSWORD='"hello"'
   -D WITH_MQTT_BRIDGE=1
   -D MAX_NEIGHBOURS=50
-  -D MAX_MQTT_BROKERS=3
   -D MQTT_MAX_PACKET_SIZE=1024
   -D MQTT_DEBUG=1
 ;  -D MESH_PACKET_LOGGING=1
@@ -359,7 +356,6 @@ build_flags =
   -D ROOM_PASSWORD='"hello"'
   -D WITH_MQTT_BRIDGE=1
   -D MAX_NEIGHBOURS=50
-  -D MAX_MQTT_BROKERS=3
   -D MQTT_MAX_PACKET_SIZE=1024
   -D MQTT_DEBUG=1
 ;  -D MESH_PACKET_LOGGING=1

variants/lilygo_t3s3/platformio.ini

@@ -116,7 +116,6 @@ build_flags =
   -D ADMIN_PASSWORD='"password"'
   -D MAX_NEIGHBOURS=50
   -D WITH_MQTT_BRIDGE=1
-  -D MAX_MQTT_BROKERS=3
   -D MQTT_MAX_PACKET_SIZE=1024
 ;  -D MQTT_DEBUG=1
 ;  -D MESH_PACKET_LOGGING=1
@@ -162,7 +161,6 @@ build_flags =
   -D ROOM_PASSWORD='"hello"'
   -D WITH_MQTT_BRIDGE=1
   -D MAX_NEIGHBOURS=50
-  -D MAX_MQTT_BROKERS=3
   -D MQTT_MAX_PACKET_SIZE=1024
   -D MQTT_DEBUG=1
 ;  -D MESH_PACKET_LOGGING=1

variants/lilygo_tbeam_1w/platformio.ini

@@ -210,7 +210,6 @@ build_flags =
   -D ADMIN_PASSWORD='"password"'
   -D MAX_NEIGHBOURS=50
   -D WITH_MQTT_BRIDGE=1
-  -D MAX_MQTT_BROKERS=3
   -D MQTT_MAX_PACKET_SIZE=1024
   -D PERSISTANT_GPS=1
   -D ENV_SKIP_GPS_DETECT=1
@@ -257,7 +256,6 @@ build_flags =
   -D ROOM_PASSWORD='"hello"'
   -D WITH_MQTT_BRIDGE=1
   -D MAX_NEIGHBOURS=50
-  -D MAX_MQTT_BROKERS=3
   -D MQTT_MAX_PACKET_SIZE=1024
   -D PERSISTANT_GPS=1
   -D ENV_SKIP_GPS_DETECT=1

variants/lilygo_tbeam_SX1262/platformio.ini

@@ -161,7 +161,6 @@ build_flags =
   -D ADMIN_PASSWORD='"password"'
   -D MAX_NEIGHBOURS=50
   -D WITH_MQTT_BRIDGE=1
-  -D MAX_MQTT_BROKERS=3
   -D MQTT_MAX_PACKET_SIZE=1024
 ;  -D MQTT_DEBUG=1
 ;  -D MESH_PACKET_LOGGING=1
@@ -204,7 +203,6 @@ build_flags =
   -D ROOM_PASSWORD='"hello"'
   -D WITH_MQTT_BRIDGE=1
   -D MAX_NEIGHBOURS=50
-  -D MAX_MQTT_BROKERS=3
   -D MQTT_MAX_PACKET_SIZE=1024
   -D MQTT_DEBUG=1
 ;  -D MESH_PACKET_LOGGING=1

variants/lilygo_tbeam_SX1276/platformio.ini

@@ -160,7 +160,6 @@ build_flags =
   -D MAX_NEIGHBOURS=50
   -D PERSISTANT_GPS=1
   -D WITH_MQTT_BRIDGE=1
-  -D MAX_MQTT_BROKERS=3
   -D MQTT_MAX_PACKET_SIZE=1024
 ;  -D MQTT_DEBUG=1
 ;  -D MESH_PACKET_LOGGING=1
@@ -203,7 +202,6 @@ build_flags =
   -D ROOM_PASSWORD='"hello"'
   -D WITH_MQTT_BRIDGE=1
   -D MAX_NEIGHBOURS=50
-  -D MAX_MQTT_BROKERS=3
   -D MQTT_MAX_PACKET_SIZE=1024
   -D MQTT_DEBUG=1
 ;  -D MESH_PACKET_LOGGING=1

variants/lilygo_tbeam_supreme_SX1262/platformio.ini

@@ -121,7 +121,6 @@ build_flags =
   -D ADMIN_PASSWORD='"password"'
   -D MAX_NEIGHBOURS=50
   -D WITH_MQTT_BRIDGE=1
-  -D MAX_MQTT_BROKERS=3
   -D MQTT_MAX_PACKET_SIZE=1024
   -D MQTT_DEBUG=1
   -D MESH_PACKET_LOGGING=1
@@ -164,7 +163,6 @@ build_flags =
   -D ROOM_PASSWORD='"hello"'
   -D WITH_MQTT_BRIDGE=1
   -D MAX_NEIGHBOURS=50
-  -D MAX_MQTT_BROKERS=3
   -D MQTT_MAX_PACKET_SIZE=1024
   -D MQTT_DEBUG=1
 ;  -D MESH_PACKET_LOGGING=1

variants/lilygo_tlora_v2_1/platformio.ini

@@ -167,7 +167,6 @@ build_flags =
   -D ADMIN_PASSWORD='"password"'
   -D MAX_NEIGHBOURS=50
   -D WITH_MQTT_BRIDGE=1
-  -D MAX_MQTT_BROKERS=3
   -D MQTT_MAX_PACKET_SIZE=1024
   -D MQTT_TASK_STACK_SIZE=16384
   -D ESP32_CPU_FREQ=240
@@ -213,7 +212,6 @@ build_flags =
   -D ADMIN_PASSWORD='"password"'
   -D ROOM_PASSWORD='"hello"'
   -D WITH_MQTT_BRIDGE=1
-  -D MAX_MQTT_BROKERS=3
   -D MQTT_MAX_PACKET_SIZE=1024
   -D MQTT_TASK_STACK_SIZE=16384
   -D ESP32_CPU_FREQ=240

variants/rak3112/platformio.ini

@@ -116,7 +116,6 @@ build_flags =
   -D WITH_MQTT_BRIDGE=1
 ; Non-PSRAM board: neighbors table costs ~4 KB of internal DRAM (see MQTTBridge.h).
   -D MQTT_NEIGHBORS_WITHOUT_PSRAM=1
-  -D MAX_MQTT_BROKERS=3
   -D MQTT_MAX_PACKET_SIZE=1024
   -D MQTT_DEBUG=1
   -D MESH_PACKET_LOGGING=1
@@ -185,7 +184,6 @@ build_flags =
 ; Non-PSRAM board: neighbors table costs ~4 KB of internal DRAM (see MQTTBridge.h).
   -D MQTT_NEIGHBORS_WITHOUT_PSRAM=1
   -D MAX_NEIGHBOURS=50
-  -D MAX_MQTT_BROKERS=3
   -D MQTT_MAX_PACKET_SIZE=1024
   -D MQTT_DEBUG=1
 ;  -D MESH_PACKET_LOGGING=1

variants/station_g2/platformio.ini

@@ -188,7 +188,6 @@ build_flags =
   -D ROOM_PASSWORD='"hello"'
   -D WITH_MQTT_BRIDGE=1
   -D MAX_NEIGHBOURS=50
-  -D MAX_MQTT_BROKERS=3
   -D MQTT_MAX_PACKET_SIZE=1024
   -D MQTT_DEBUG=1
 ;  -D MESH_PACKET_LOGGING=1
@@ -315,7 +314,6 @@ build_flags =
   -D ADMIN_PASSWORD='"password"'
   -D MAX_NEIGHBOURS=50
   -D WITH_MQTT_BRIDGE=1
-  -D MAX_MQTT_BROKERS=3
   -D MQTT_MAX_PACKET_SIZE=1024
   -D MQTT_DEBUG=1
 #  -D MESH_PACKET_LOGGING=1

variants/station_g3_esp32/platformio.ini

@@ -176,7 +176,6 @@ build_flags =
   -D ADMIN_PASSWORD='"password"'
   -D MAX_NEIGHBOURS=50
   -D WITH_MQTT_BRIDGE=1
-  -D MAX_MQTT_BROKERS=3
   -D MQTT_MAX_PACKET_SIZE=1024
   -D MQTT_DEBUG=1
 #  -D MESH_PACKET_LOGGING=1
@@ -224,7 +223,6 @@ build_flags =
   -D ROOM_PASSWORD='"hello"'
   -D WITH_MQTT_BRIDGE=1
   -D MAX_NEIGHBOURS=50
-  -D MAX_MQTT_BROKERS=3
   -D MQTT_MAX_PACKET_SIZE=1024
   -D MQTT_DEBUG=1
 ;  -D MESH_PACKET_LOGGING=1

variants/thinknode_m7/platformio.ini

@@ -198,7 +198,6 @@ build_flags =
   -D ADMIN_PASSWORD='"password"'
   -D MAX_NEIGHBOURS=50
   -D WITH_MQTT_BRIDGE=1
-  -D MAX_MQTT_BROKERS=3
   -D MQTT_MAX_PACKET_SIZE=1024
   -D MQTT_DEBUG=1
 #  -D MESH_PACKET_LOGGING=1
@@ -244,7 +243,6 @@ build_flags =
   -D ROOM_PASSWORD='"hello"'
   -D MAX_NEIGHBOURS=50
   -D WITH_MQTT_BRIDGE=1
-  -D MAX_MQTT_BROKERS=3
   -D MQTT_MAX_PACKET_SIZE=1024
   -D MQTT_DEBUG=1
 #  -D MESH_PACKET_LOGGING=1

variants/xiao_s3_wio/platformio.ini

@@ -111,7 +111,6 @@ build_flags =
   -D ADMIN_PASSWORD='"password"'
   -D MAX_NEIGHBOURS=50
   -D WITH_MQTT_BRIDGE=1
-  -D MAX_MQTT_BROKERS=3
   -D MQTT_MAX_PACKET_SIZE=1024
   -D MQTT_DEBUG=1
   -D MESH_PACKET_LOGGING=1
@@ -155,7 +154,6 @@ build_flags =
   -D ROOM_PASSWORD='"hello"'
   -D WITH_MQTT_BRIDGE=1
   -D MAX_NEIGHBOURS=50
-  -D MAX_MQTT_BROKERS=3
   -D MQTT_MAX_PACKET_SIZE=1024
   -D MQTT_DEBUG=1
 ;  -D MESH_PACKET_LOGGING=1