Upgrade: Clean tasks from users who have been deleted
Through #5783, it has come to our attention that the UserManager::delete_user() method didn't include a mechanism to delete tasks from users who have been removed from the system.
As such, we have updated work.lib.php (see #5783) to include a JOIN on the user table. This, however, is inefficient. Now that #5783 has implemented the right mechanism in delete_user(), remove all the "ghost" assignments from the c_student_publication table during the upgrade from 1.9 to 1.10.
Then remove the check on the user table from work.lib.php.