Copter: remove broken support for do-change-alt

This command was not implemented fully removing this code resolves a
compiler warning

ArduCopter/GCS_Mavlink.pde

@@ -922,8 +922,6 @@ void GCS_MAVLINK::handle_change_alt_request(AP_Mission::Mission_Command &cmd)
     }
 
     // To-Do: update target altitude for loiter or waypoint controller depending upon nav mode
-    // similar to how do_change_alt works
-    wp_nav.set_desired_alt(cmd.content.location.alt);
 }
 
 void GCS_MAVLINK::handleMessage(mavlink_message_t* msg)

ArduCopter/commands_logic.pde

@@ -763,29 +763,6 @@ static void do_wait_delay(const AP_Mission::Mission_Command& cmd)
 
 static void do_change_alt(const AP_Mission::Mission_Command& cmd)
 {
-    // adjust target appropriately for each nav mode
-    if (control_mode == AUTO) {
-        switch (auto_mode) {
-        case Auto_TakeOff:
-            // To-Do: adjust waypoint target altitude to new provided altitude
-            break;
-        case Auto_WP:
-        case Auto_Spline:
-            // To-Do; reset origin to current location + stopping distance at new altitude
-            break;
-        case Auto_Land:
-        case Auto_RTL:
-            // ignore altitude
-            break;
-        case Auto_CircleMoveToEdge:
-        case Auto_Circle:
-            // move circle altitude up to target (we will need to store this target in circle class)
-            break;
-        case Auto_NavGuided:
-            // ignore altitude
-            break;
-        }
-    }
     // To-Do: store desired altitude in a variable so that it can be verified later
 }