Allow feature gate removal through the API

What does this MR do?

Allows admins to clean up the list of feature gates in the database.

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?

None, as far as I know