Plane: update comment and remove redundant work

- update_flight_stage is already called within restart_landing_sequence()

ArduPlane/ArduPlane.cpp

@@ -525,7 +525,8 @@ void Plane::handle_auto_mode(void)
     uint16_t nav_cmd_id;
 
     if (mission.state() != AP_Mission::MISSION_RUNNING) {
-        // this should never be reached
+        // this could happen if AP_Landing::restart_landing_sequence() returns false which would only happen if:
+        // restart_landing_sequence() is called when not executing a NAV_LAND or there is no previous nav point
         set_mode(RTL, MODE_REASON_MISSION_END);
         GCS_MAVLINK::send_statustext_all(MAV_SEVERITY_INFO, "Aircraft in auto without a running mission");
         return;

ArduPlane/system.cpp

@@ -546,9 +546,6 @@ void Plane::exit_mode(enum FlightMode mode)
             if (mission.get_current_nav_cmd().id == MAV_CMD_NAV_LAND)
             {
                 landing.restart_landing_sequence();
-
-                // exit landing stages if we're no longer executing NAV_LAND
-                update_flight_stage();
             }
         }
         auto_state.started_flying_in_auto_ms = 0;