Sub: remove definition of Devo constructor

ArduSub/GCS_Sub.cpp

@@ -133,5 +133,4 @@ void GCS_Sub::update_sensor_status_flags()
 }
 
 // avoid building/linking Devo:
-AP_DEVO_Telem::AP_DEVO_Telem() {}
 void AP_DEVO_Telem::init() {};