Remove projects_preloader_fix feature flag

What does this MR do and why?

Remove projects_preloader_fix feature flag. This feature flag has been default enabled.

The performance improvements were realized so the FF can be removed now.

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How to set up and validate locally

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

MR acceptance checklist

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

Closes #378858 (closed)