Sub: use gps singleton for GCS functions

ArduSub/GCS_Mavlink.cpp

@@ -1628,11 +1628,6 @@ AP_Mission *GCS_MAVLINK_Sub::get_mission()
     return &sub.mission;
 }
 
-AP_GPS *GCS_MAVLINK_Sub::get_gps() const
-{
-    return &sub.gps;
-}
-
 AP_Camera *GCS_MAVLINK_Sub::get_camera() const
 {
 #if CAMERA == ENABLED

ArduSub/GCS_Mavlink.h

@@ -19,7 +19,6 @@ protected:
     AP_Rally *get_rally() const override;
     AP_Camera *get_camera() const override;
     AP_ServoRelayEvents *get_servorelayevents() const override;
-    AP_GPS *get_gps() const override;
     const AP_FWVersion &get_fwver() const override;
     void set_ekf_origin(const Location& loc) override;