Remove `personal_access_tokens_scoped_to_projects` feature flag

What does this MR do and why?

Related to #322187 (closed)

In !54617 (merged) we introduced a project selector to the personal access token form. This selector would allow you to scope a personal access token to a project. This was added before the backend supported it because we had frontend capacity. Due to capacity restrictions the backend was never completed and product has decided to go in a different direction. See #15666 (comment 915975220)

This MR removes the personal_access_tokens_scoped_to_projects feature flag and corresponding code as it is no longer needed.

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