Remove unused DiffService#patch RPC
What does this MR do?
Remove an unused RPC This RPC has been replaced by the RawPatch RPC, which is called by workhorse as creating a patch could take a long time. The CommitPatch RPC has no callsites in this codebase, which allows us to remove it. Part of: https://gitlab.com/gitlab-org/gitaly/issues/2095
Edited by Zeger-Jan van de Weg