ArduSub: provide default implemenation of handle_change_alt_request

The TODO items to actually implement these are almost 6 years old.
Since then these methods have been burning space.

This doesn't even make sense for several vehicles, so a default
implementation which does nothing seems OK.

ArduSub/GCS_Mavlink.cpp

@@ -407,16 +407,6 @@ bool GCS_MAVLINK_Sub::handle_guided_request(AP_Mission::Mission_Command &cmd)
     return sub.do_guided(cmd);
 }
 
-void GCS_MAVLINK_Sub::handle_change_alt_request(AP_Mission::Mission_Command &cmd)
-{
-    // add home alt if needed
-    if (cmd.content.location.relative_alt) {
-        cmd.content.location.alt += sub.ahrs.get_home().alt;
-    }
-
-    // To-Do: update target altitude for loiter or waypoint controller depending upon nav mode
-}
-
 MAV_RESULT GCS_MAVLINK_Sub::_handle_command_preflight_calibration_baro()
 {
     if (sub.motors.armed()) {

ArduSub/GCS_Mavlink.h

@@ -43,7 +43,6 @@ private:
 
     void handleMessage(const mavlink_message_t &msg) override;
     bool handle_guided_request(AP_Mission::Mission_Command &cmd) override;
-    void handle_change_alt_request(AP_Mission::Mission_Command &cmd) override;
     bool try_send_message(enum ap_message id) override;
 
     bool send_info(void);