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.
Errors:
```
SassError: SassError: compound selectors may no longer be extended.
Consider `@extend a, :hover` instead.
See http://bit.ly/ExtendCompound for details.
```
