fixes #3591 : revert http://develop.studip.de/trac/changeset/25409/trunk/app/controllers/admin/sem_classes.php from [25409] regarding the sem-class-controller.

git-svn-id: svn://develop.studip.de/studip/trunk@27020 c5b85792-5220-0410-ad23-ed5b6b8ad999