Remove startup scss files

What does this MR do and why?

Remove startup scss files

The files are completely unused since !124406 (merged)

This removes them.

Related MRs

Note, there are two related MRs:

• !135232 (merged) (needed for this one) which stops compiling the assets
• !135233 (merged) (independent) which removes the startup css generator code

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)

How to set up and validate locally

N/A

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.