Remove CI Minutes FFs from the codebase

What does this MR do?

Drop :ci_minutes_enforce_quota_for_public_projects, :ci_minutes_track_for_public_projects Feature Flags and legacy code paths from the codebase.

⚠ We enabled both Feature Flags on 25.05.2020, it is safe to wait a couple of days (or a week) before dropping the code entirely, to have an ability to disable the FFs in case of emergency.

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

N/A

Security

N/A

Related to #218434 (closed)