[TASK] Cleanup default content CSS for CKEditor5

Most of the CSS is already available through the
Backend CSS. We are removing most of the outdated
special stylings from previous CKEditor versions.

Resolves: #99466
Releases: main

Change-Id: I4a6ab0e289d526b4fb4645256b15968c61f2de57
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/77259


Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>
Tested-by: core-ci <typo3@b13.com>