Calculating repository checksums executed by Gitaly

What does this MR do?

Remove the alternate implementation for calculating checksums.

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)

What are the relevant issue numbers?

Part of gitaly#1105 (closed)

Also closes gitaly#1105 (closed)

Also closes https://gitlab.com/gitlab-org/gitlab-ce/issues/46293