Tracker: move serial_control case to base class

AntennaTracker/GCS_Mavlink.cpp

@@ -740,10 +740,6 @@ mission_failed:
         break;
     }
 
-    case MAVLINK_MSG_ID_SERIAL_CONTROL:
-        handle_serial_control(msg, tracker.gps);
-        break;
-
     case MAVLINK_MSG_ID_AUTOPILOT_VERSION_REQUEST:
         send_autopilot_version(FIRMWARE_VERSION);
         break;