Stop compiling startup css in the asset pipeline

What does this MR do and why?

Stop compiling startup css in the asset pipeline

Since !124406 (merged) they are unused.

Remove startup CSS loader code

Since !124406 (merged) we do not use this code any longer

Related MRs

Note, there are two related MRs:

• !135233 (merged) (independent) which removes the startup css generator code
• !124404 (merged) (depends on this) removing the SCSS files

Screenshots or screen recordings

N/A

How to set up and validate locally

1. Remove public/assets/startup/
2. bin/rails assets:precompile should not compile public/assets/startup/ files any longer

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.