Remove no-longer-used servlets/components (#2868)

With GKE, we don't need the individual servlets because the services
aren't partitioned out the same way they were in GAE.

We keep FrontendServlet and BackendServlet around for now as they serve
as the backbone for the local RegistryTestServer (for testing things
like the console).

did some cursory tests on alpha and things seem to be unaffected -- I
was able to curl RDAP (pubapi) and create domains

Author: gbrodman