Completes migration to the new admin v3 styling system by:
- Removing conditional stylesheet inclusion in admin head
- Deleting all legacy admin style files and components
- Making admin-style-v3 the default and only stylesheet
This change reduces maintenance overhead and simplifies the admin styling codebase by removing the old styling system that was being conditionally loaded based on feature flags.
Tags' rules are still coming from the old admin styles hence had to add
the new (admin_v3) border variable to the old one.
Has a hard coded colour value of #2e3132 as it has no access
to the new colours.
There were all sorts of dependencies hiding in there, and it was overwriting some of the admin variables. Better to keep things separate and simple.
I discovered that there was one legitimate reason for importing them though: for styling the trix editor. I can't think of a good way to safely import them without affecting the other variables, so I just hardcoded the colours for now.
