Remove branch_push_merge_commit_analyze feature flag

What does this MR do?

We added this flag in gitlab-foss!22794 (merged) to be cautious. We've never toggled it, so we can just delete it.

Part of #251080 (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