navigator/mavlink: fixes for mission item reached message (#5486)

* navigator/mavlink: always send last reached item

Since we can't rely on mavlink that every message arrives, it makes
sense to continuously send the last reached waypoint.

* navigator: don't report reached for takeoff

If a takeoff waypoint has been inserted, we should not report that we
reached a mavlink mission item because we actually have not.

src/modules/navigator/navigator_main.cpp

@@ -878,7 +878,6 @@ Navigator::publish_mission_result()
 	}
 
 	/* reset some of the flags */
-	_mission_result.seq_reached = false;
 	_mission_result.seq_current = 0;
 	_mission_result.item_do_jump_changed = false;
 	_mission_result.item_changed_index = 0;