Remove github_import_lock_user_finder feature flag

What does this MR do and why?

Removes the github_import_lock_user_finder that has been by default enabled on .com since April, 8th 2024.

MR acceptance checklist

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Screenshots or screen recordings

N/A

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Related to #438336 (closed)