message: mission plugin: Clear old mission items
  Before importing mission items from a QGroundControl plan, we've
  to clear old ones; otherwise they would reside along with new mission
  items, which is not expected. This commit clears them before importing
  mission items.
diff: @@ -954,6 +954,9 @@ MissionImpl::import_qgroundcontrol_mission(Mission::mission_items_t &mission_ite
  return Mission::Result::FAILED_TO_PARSE_QGC_PLAN;
  }
  + // Clear old mission items
  + mission_items.clear();
  +
  // Import mission items
  return import_mission_items(mission_items, parsed_plan);
  }
  