Clean-up libraries-overrides in default_admin.info.yml

Problem/Motivation

This is a follow-up issue to #3612540: [regression] Fix Toolbar with Default Admin where we remove the reference to a non-existing css-file which then broke toolbar styling. It became obvious, that a few more references are also incorrect due to CSS refactoring in default_admin.

Steps to reproduce

Proposed resolution

Let's go through all the overrides, verify that they're still valid, or remove otherwise. This should wait for all the CSS refactoring to be completed, reviewed, and committed.

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet