Remove require_notes_last_fetched_at feature flag

What does this MR do and why?

This makes the X-Last-Fetched-At header required for the notes polling endpoint.

There is no visible change to the user because this header is always included when our frontend makes the request. This is just to prevent bots and scrapers from unintentionally hitting this endpoint.

This has been enabled on GitLab.com for ~2 days.

MR acceptance checklist

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

Related to #419829 (closed)