[BUGFIX] Fix styling in AdminPanel

Since Bootstrap 5, the legacy.scss is also included
in the Admin Panel, which breaks the layout.

The change removes the legacy.scss and mixins inclusion,
and makes the Admin Panel work again.

Resolves: #93183
Releases: master
Change-Id: I0b6901be5a97cd77d7920cf641ccb659bd68d7a8
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/67973


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Kevin Appelt <kevin.appelt@icloud.com>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Kevin Appelt <kevin.appelt@icloud.com>
Reviewed-by: Benni Mack <benni@typo3.org>