Remove gldropdown_branches feature flag [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

This removes the gldropdown_branches feature flag

This completes the rollout of #326549 (closed)

Screenshots (strongly suggested)

N/A

Does this MR meet the acceptance criteria?

Conformity

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• [-] Documentation (if required)
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• [-] Database guides
• [-] Separation of EE specific content