Tracker: use camera singleton to get camera rather than callback

AntennaTracker/GCS_Mavlink.h

@@ -16,7 +16,6 @@ protected:
 
     AP_Mission *get_mission() override { return nullptr; };
     AP_Rally *get_rally() const override { return nullptr; };
-    AP_Camera *get_camera() const override { return nullptr; };
 
     uint8_t sysid_my_gcs() const override;