Remove FF hide_merge_requests_from_banned_users

What does this MR do and why?

Removes the hide_merge_requests_from_banned_users feature flag as it is now enabled across all environments.

I also verified the feature flag status for self-managed customers using the Tableau dashboard and did not find any instance where the flag is disabled.

References

#520687 (closed) #410671 (closed)

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Related to #520687 (closed)