Disable startup CSS

What does this MR do and why?

Remove remove_startup_css feature flag and checks. Feature flag has been enabled on Gitlab for 2 months.

MR that deletes all startupCSS code: !124404 (merged)

Why are you doing this?

There is a small performance benefit to most users, and we get to remove 10k lines of code.

Some discussions and science in gitlab-org/frontend/rfcs#92 (comment 1187261050)

MR acceptance checklist

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