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

variants/heltec_tracker/platformio.ini

@@ -116,7 +116,6 @@ 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 = ${Heltec_tracker_base.build_src_filter}