messages: remove associated tags on delete, fixes #4613

git-svn-id: svn://develop.studip.de/studip/trunk@31417 c5b85792-5220-0410-ad23-ed5b6b8ad999
messages: relocate tag deletion from message deletion to message user connection deletion, refines [31417], re #4613

git-svn-id: svn://develop.studip.de/studip/trunk@31418 c5b85792-5220-0410-ad23-ed5b6b8ad999