API: Remove `DELETE projects/:id/deploy_keys/:key_id/disable`

What does this MR do?

This removes a duplicated API endpoint to remove a deploy key from a project.

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #28286 (closed)