Remove legacy fork job support from RepositoryForkWorker

This is one of the remaining spots in gitlab-ce where we use direct access to repo storage. We have left-over transition code, to handle old sidekiq jobs during a deploy. It should be possible to remove the legacy job support -- but do double check this yourself.

https://gitlab.com/gitlab-org/gitlab-ce/blob/f67eb5a1ab6ec75aae24db4b3b98e23faafa6de6/app/workers/repository_fork_worker.rb#L21-32

---

Stages:

• RPC Design: Create Issue

• ~"Client Implementation": Create Issue

• Server Implementation: Create Issue

• ~"Acceptance Testing": Create Issue

RPC Endpoints:

• Endpoint::Name

Known Client Routes:

• Known client endpoints

---