Remove the single-file file-by-file feature flag

What does this MR do and why?

Resolves FF #403571 (closed)

This MR removes the single_file_file_by_file feature flag that was fully enabled for .com on production on 2023-04-21.

After a minimum of 24 hours, that feature is considered enabled and rolled out, so this MR removes the feature flag.

MR acceptance checklist

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