Remove Rugged::Walk implementation of #log

What does this MR do?

This is the second shot at removing this code, after https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/16525 got reverted in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/16676.

We had to revert at the time because we were diverting requests to a code path that did not handle alternate object directories well in gitaly-ruby. This has been resolved in Gitaly 0.76.0, via https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/16715.

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-qa manual pipeline job)

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