Resolve "Group types that are managed by some app (e.g. cursus' "school class") can't be selected as timetable group type"

Closes #245 (closed)

@hansegucker could you have a look?

aleksis/apps/chronos/preferences.py

@@ -127,9 +127,6 @@ class GroupTypesTimetables(ModelMultipleChoicePreference):
     verbose_name = _("Group types to show in timetables")
     help_text = _("If you leave it empty, all groups will be shown.")
 
-    def get_queryset(self):
-        return GroupType.objects.managed_and_unmanaged()
-
 
 @site_preferences_registry.register
 class LessonEventFeedColor(StringPreference):