REST UI module should not be rebuilding routes

This snippet exists in two places (and has the necessary service injection with all the associated boilerplate):

     // Rebuild routing cache.
     $this->routeBuilder->rebuild();

This should not be necessary. But indeed it currently is. #2815845: Importing (deploying) REST resource config entities should automatically do the necessary route rebuilding will fix this in core, and then the code in REST UI can be simplified.