Remove 'Repo' prefix from API entities

What does this MR do?

1. Removes the Repo prefix from API entities.
2. Removes the hack added in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9351

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

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/28417