Remove `ci_pipeline_open_merge_requests` FF [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

This feature is stable and no longer needs a feature flag.

Closes #292727 (closed)

Does this MR meet the acceptance criteria?

Conformity

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