Trip plugin - remove constant `trips_index`

[daniel-j-h]

We used to split trips based on components. By now we only ever return a single trip or error out with NoTrips if the coordinates are not in a single component.

The trips_index is always 0 now - we only ever return a single trip.

https://github.com/Project-OSRM/osrm-backend/blob/master/docs/http.md#response

Remove trips_index from the backend and the node bindings and adapt docs.

Breaking change: needs to wait for v6.

cc @chaupow @ghoshkaj