ArduCopter: 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.

ArduCopter/GCS_Mavlink.cpp

@@ -559,16 +559,6 @@ bool GCS_MAVLINK_Copter::handle_guided_request(AP_Mission::Mission_Command &cmd)
 #endif
 }
 
-void GCS_MAVLINK_Copter::handle_change_alt_request(AP_Mission::Mission_Command &cmd)
-{
-    // add home alt if needed
-    if (cmd.content.location.relative_alt) {
-        cmd.content.location.alt += copter.ahrs.get_home().alt;
-    }
-
-    // To-Do: update target altitude for loiter or waypoint controller depending upon nav mode
-}
-
 void GCS_MAVLINK_Copter::packetReceived(const mavlink_status_t &status,
                                         const mavlink_message_t &msg)
 {

ArduCopter/GCS_Mavlink.h

@@ -52,7 +52,6 @@ private:
     void handleMessage(const mavlink_message_t &msg) override;
     void handle_command_ack(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;
 
     void packetReceived(const mavlink_status_t &status,