[C2] Classes: number of courses shown in classes list is incorrect

[ywarnier]

• Create a class (main/admin/usergroups.php?action=add)
• Go to the page to add courses to the class (main/admin/add_courses_to_usergroup.php?id=1)
• Add one course to the class
• Return to the list of classes (main/admin/usergroups.php)
• The "Courses" column shows "2"
• Add another course to the class
• The "Courses" column shows "4"

Apparently each course counts twice.