Migrate GitlabProject (re)move project endpoints

What does this MR do?

Migrate GitlabProject (re)move project endpoints

Migration is done through a small refactoring, which makes us call endpoins which are performing the same actions for namespaces. Tests are added to ensure only the project is removed that should be removed.

Closes gitaly#873 (closed)

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?

gitaly#873 (closed)