Remove unnecessary index_projects_on_builds_enabled index from the projects table

Are there points in the code the reviewer needs to double check?

I'm pretty sure this should not require any downtime, but we've been surprised several time recently, so a confirmation that this migration is fine to run online would be good.

What are the relevant issue numbers?

Closes #20491 (closed)

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 style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together