Remove many unused bg- CSS classes

What does this MR do and why?

Removes 6kB from application_utilities.css. We were adding all variations of bg-success-50 -> bg-success-900 for all variants, but most were unused.

Screenshots or screen recordings

No visual changes.

How to set up and validate locally

Search for usages for any bg-* classes I missed. I used this regex: (?!gl-)bg-(primary|secondary|success|warning|danger)- The trailing hyphen is because bg-secondary still works (that comes from bootstrap).

MR acceptance checklist

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