[http-client-java] remove vanilla-related code

Back in swagger/autorest time, we have vanilla flavor for some services, e.g. storage. Now that autorest is deprecated, we can cleanup its path.

Start in the http-client-java area and search for the vanilla-related path left from the deprecated AutoRest workflow. Trace its callers and surrounding generation or client code before deciding what is safe to remove. Done means the obsolete path is removed without affecting supported services, with the existing project checks passing.

Written by the indexing model from the issue text.