如何删除谷歌地图路线方向上的默认A B标记

var directionsService = new google.maps.DirectionsService();
var directionsDisplay = new google.maps.DirectionsRenderer();

var startMarker = new google.maps.Marker({ position: start, map: map, icon: 'start.png' });
var stopMarker = new google.maps.Marker({ position: stop, map: map, icon: 'stop.png' });

directionsDisplay.setMap(map);

var request = {
 origin: start, 
 destination: stop,
 travelMode: google.maps.DirectionsTravelMode.DRIVING
};

directionsService.route(request, function(response, status) {
 if (status == google.maps.DirectionsStatus.OK) {
  directionsDisplay.setDirections(response);
 }
});
var directionsService = new google.maps.Directi