Historically, Gitaly easily ended up in states where the repository is
in a state such that many different Git commands failed. This is why
Gitaly provides a Cleanup RPC that cleans up any such data structures
which are known to cause problems, and we call it in many different
places.

There had been two main usecases for this:

    - Git is known to leave behind corrupted references in case of a
      hard shutdown because it doesn't fsync(3p) data to disk before
      renaming loose references into place. Such broken references may
      cause Git commands to fail which try to read them, and as such
      Gitaly used to perform housekeeping tasks where it walked through
      the repository to clean up any such corrupt references. Gitaly had
      to eventually remove this functionality from Cleanup though: in
      repositories hosted on NFS this was causing significant latency
      and thus caused timeouts. We thus don't do this anymore since
      7a1d224d0 (repository: Remove housekeeping from Cleanup RPC,
      2021-05-17).

    - Many of Gitaly's RPCs used to create worktrees to perform various
      operations. In some cases worktrees were pruned, but left behind
      broken references which as a result again caused corrupted
      references. The Cleanup RPC thus prunes any such per-worktree
      references and cleans up pointers to already-removed worktrees.
      Gitaly has since converted all RPCs except UserApplyPatch to not
      use worktrees anymore, and this remaining RPC is effectively
      unused: this week we had two calls of it across all of gitlab.com.

It is thus safe to say that the Cleanup RPC is effectively useless and
doesn't do anything. It doesn't clean up corrupted references, and
because we don't create worktrees we wouldn't ever need to clean them up
either. Furthermore, both of those actions are performed by other RPCs,
namely GarbageCollect and OptimizeRepository. The former one is
frequently executed by Rails, while the latter one is automatically
executed during our daily maintenance window on all repositories.

Let's thus stop calling Cleanup altogether such that its RPC can be
properly deprecated and removed in Gitaly.

Changelog: removed