Remove merge_request_approval_policies_target_branch_matching FF

What does this MR do and why?

Removes the merge_request_approval_policies_target_branch_matching feature flag introduced by !191989 (merged).

References

• Feature flag is globally enabled on GitLab.com: https://gitlab.slack.com/archives/C101F3796/p1752583562871189
• #529997 (closed)
• #528852 (closed)

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