Remove startup CSS generator code

What does this MR do and why?

Remove startup CSS generator code

We do not rely on startup CSS any longer: !124406 (merged)

So we do not need to update the files. This also removes unused dev dependencies.

Related MRs

Note, there are two related MRs:

• !135232 (merged) (independent) which stops compiling the assets
• !124404 (merged) (depends on !135232 (merged)) removing the SCSS files

Screenshots or screen recordings

N/A

How to set up and validate locally

N/A

MR acceptance checklist

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