Assignment is not deleted when student group changes

[jprk]

If a student changes hers/his student group after assignments were generated, the original assinment record remains in place. As a result, the record in assignmnts table referes to serveral assignments for the student.

[jprk]

We need to execute something like

DELETE FROM assignmnts WHERE student_id=425678 AND subtask_id=1234 AND year=2018

after every removal of a student from a group. This happens in StudentGroupBean::doRealDelete() and is part of AssignmentsBean::dbReplace().