[TASK] Unify descriptions for fixture classes of abstract classes

There is no point repeating the name of the extended class in the
class PHPDoc comment.

This makes creating new fixture classes less of a hassle.

This is a separate change in order to make the other changes that
get rid of `getMockForAbstractClass` shorter and more focused,
following the "one kind of change per change" principle.

Resolves: #101665
Related: #101601
Releases: main, 12.4
Change-Id: I7ee95e63149ff90c6f5817438e5b4634d04f4e93
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/80947


Tested-by: Benjamin Franzke <ben@bnf.dev>
Reviewed-by: Benjamin Franzke <ben@bnf.dev>
Tested-by: core-ci <typo3@b13.com>