Remove WITH_ESPNOW_BRIDGE_SECRET definition from platformio.ini files and update documentation to use _prefs->bridge_secret

variants/station_g2/platformio.ini

@@ -76,7 +76,6 @@ build_flags =
   -D ADMIN_PASSWORD='"password"'
   -D MAX_NEIGHBOURS=8
   -D WITH_ESPNOW_BRIDGE=1
-  -D WITH_ESPNOW_BRIDGE_SECRET='"shared-secret"'
 ;  -D MESH_PACKET_LOGGING=1
 ;  -D MESH_DEBUG=1
 build_src_filter = ${Station_G2.build_src_filter}
@@ -139,7 +138,6 @@ build_flags =
   -D MESH_PACKET_LOGGING=1
   -D SX126X_RX_BOOSTED_GAIN=1
   -D WITH_ESPNOW_BRIDGE=1
-  -D WITH_ESPNOW_BRIDGE_SECRET='"shared-secret"'
 ;  -D MESH_DEBUG=1
 build_src_filter = ${Station_G2.build_src_filter}
   +<helpers/bridges/ESPNowBridge.cpp>