Disable Waypoints on Google Maps Directions API - google-maps

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>