Remove `user_pat_rest_api` FF

What does this MR do and why?

Remove user_pat_rest_api FF

This FF has been temporarily introduced in 16.5 and has never been released to self-managed.

Refs [Feature flag] Rollout of `user_pat_rest_api` (#425967 - closed)

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.