Sub: GCS_MAVLink now uses Mission singleton

ArduSub/GCS_Mavlink.cpp

@@ -1162,11 +1162,6 @@ void Sub::mavlink_delay_cb()
     DataFlash.EnableWrites(true);
 }
 
-AP_Mission *GCS_MAVLINK_Sub::get_mission()
-{
-    return &sub.mission;
-}
-
 AP_Rally *GCS_MAVLINK_Sub::get_rally() const
 {
 #if AC_RALLY == ENABLED

ArduSub/GCS_Mavlink.h

@@ -12,7 +12,6 @@ protected:
         return 0;
     };
 
-    AP_Mission *get_mission() override;
     AP_Rally *get_rally() const override;
 
     MAV_RESULT handle_flight_termination(const mavlink_command_long_t &packet) override;