Make all workhorse gitaly calls opt-out, take 2

What does this MR do?

Fixes https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/18002, which was reverted. Original description:

Make all Git features in gitlab-workhorse use Gitaly by default. Administrators can still opt-out of the features affected by this MR if something is broken. We don't expect this to be needed because we have been using these Gitaly calls in Workhorse for months without trouble on gitlab.com.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

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