Tools: 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.

Tools/AP_Periph/GCS_MAVLink.h

@@ -31,7 +31,6 @@ private:
     uint32_t telem_delay() const override { return 0; }
     void handleMessage(const mavlink_message_t &msg) override { handle_common_message(msg); }
     bool handle_guided_request(AP_Mission::Mission_Command &cmd) override { return true; }
-    void handle_change_alt_request(AP_Mission::Mission_Command &cmd) override {}
     MAV_RESULT handle_preflight_reboot(const mavlink_command_long_t &packet) override;
 
 protected: