Clean up FF for security policy approval rule creation

What does this MR do and why?

Clean up the merge_request_approval_policies_create_approval_rules_without_protected_branches feature flag introduced by !191989 (merged).

References

• The feature flag is globally enabled on GitLab.com: https://gitlab.slack.com/archives/C101F3796/p1754292376130659
• #547858 (closed)
• #529997 (closed)

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #547858 (closed)