AntennaTracker: 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.

AntennaTracker/GCS_Mavlink.cpp

@@ -169,11 +169,6 @@ bool GCS_MAVLINK_Tracker::handle_guided_request(AP_Mission::Mission_Command&)
     return false;
 }
 
-void GCS_MAVLINK_Tracker::handle_change_alt_request(AP_Mission::Mission_Command&)
-{
-    // do nothing
-}
-
 /*
   default stream rates to 1Hz
  */

AntennaTracker/GCS_Mavlink.h

@@ -38,7 +38,6 @@ private:
     void mavlink_check_target(const mavlink_message_t &msg);
     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;
     void handle_set_attitude_target(const mavlink_message_t &msg);
 
     void send_global_position_int() override;