[BUGFIX] Remove obsolete `diff-[r|g]` classes

The CSS classes `.diff-[r|g]` were used in the old `DiffUtility` that
was removed in the v7 development cycle, but it was missed to remove the
CSS as well. This patch removes the CSS classes and it's remaining
reference in a language label.

Resolves: #103335
Releases: main
Change-Id: Ie1fb7122fc906b160cbda7bd96979bc4c97549a6
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/83326


Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Andreas Kienast <a.fernandez@scripting-base.de>
Tested-by: Oliver Hader <oliver.hader@typo3.org>
Reviewed-by: Andreas Kienast <a.fernandez@scripting-base.de>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Benni Mack <benni@typo3.org>