Remove waitForCssLoaded function

What does this MR do and why?

Remove waitForCssLoaded function

Part of Remove startup scss files (!124404 - merged), which has already been disabled so this function was just resolving immediately.

This was like wrapping all the calls in a Promise.resolve(() => { , so removing should not have any effect.

Screenshots or screen recordings

No visual changes

How to set up and validate locally

Things worth validating:

• autosize of text fields (function is called immediately rather than waiting for a promise.
• source editor initial layout is correctly sized

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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