(TK-52) Remove default endpoint requirement

This PR removes the default endpoint requirement from the webrouting service. If no route-id is specified when calling a service function, the operation will still try to use the route with id of `:default`. However, it is no longer a requirement that this key be present.

It didn't seem worth it to allow the user to specify a default web route in a way similar to how the user would specify a default server, since a web route is intended to have something added to it only once.
`(get-in config [svc])` can be simplified to `(get config svc)`