Disable Waypoints on Google Maps Directions API

Question

I am trying to create a service for the user to calculate the distance between two points using Google maps "draggable directions" as per this link.

Draggable Directions

I want to disable or disallow the user to add waypoints.

My code is as follows:

<script>
      function initMap() {
        var map = new google.maps.Map(document.getElementById('map'), {
          zoom: 4,
          center: {lat: -24.345, lng: 134.46}  // Australia.
        });

        var directionsService = new google.maps.DirectionsService;
        var directionsDisplay = new google.maps.DirectionsRenderer({
          draggable: true,
          map: map,
          panel: document.getElementById('right-panel')
        });

        directionsDisplay.addListener('directions_changed', function() {
          computeTotalDistance(directionsDisplay.getDirections());
        });

        displayRoute('Perth, WA', 'Sydney, NSW', directionsService,
            directionsDisplay);
      }

      function displayRoute(origin, destination, service, display) {
        service.route({
          origin: origin,
          destination: destination,
          travelMode: 'DRIVING',
          avoidTolls: true
        }, function(response, status) {
          if (status === 'OK') {
            display.setDirections(response);
          } else {
            alert('Could not display directions due to: ' + status);
          }
        });
      }

      function computeTotalDistance(result) {
        var total = 0;
        var myroute = result.routes[0];
        for (var i = 0; i < myroute.legs.length; i++) {
          total += myroute.legs[i].distance.value;
        }
        total = total / 1000;
        document.getElementById('total').innerHTML = total + ' km';
      }
    </script>
    <script async defer
    src="https://maps.googleapis.com/maps/api/js?key=YOUR_API_KEY&callback=initMap">
    </script>