Cleanup allow_custom_root_folder_name_in_npm_upload FF

What does this MR do and why?

In Fix publishing npm package with custom root fol... (!159095 - merged), we allowed publishing NPM packages to the npm Registry if their tarballs have custom root folder name.

The change was behind a feature flag, and it was globally enabled on gitlab.com for 2 weeks now without any issues.

In this MR, we clean up the feature flag and release the fix.

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Related to #473225 (closed)