Revert "Move Repository#write_ref to Git::Repository#write_ref"

What does this MR do?

Reverts a bad client preparation https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15712 that caused https://gitlab.com/gitlab-org/gitlab-ce/issues/41143

After this is merged we need to upstream lib/gitlab/git in Gitaly as well. /cc @ahmadsherif for https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/16018

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

What are the relevant issue numbers?

Closes #41143 (closed)