show "exit course" action only for non-groupable courses, fixes #532

app/views/my_courses/_course.php

@@ -67,7 +67,7 @@
                         <?= Icon::create('door-leave+decline', 'inactive', ['title' => _("Die Teilnahme ist bindend. Bitte wenden Sie sich an die Lehrenden.")])->asImg(20) ?>
                     </a>
                     <?
-                else : ?>
+                elseif (!$values['is_group']) : ?>
                     <a href="<?= $controller->link_for('my_courses/decline', $course['seminar_id'], ['cmd' => 'suppose_to_kill']) ?>">
                         <?= Icon::create('door-leave', 'inactive', ['title' => _("aus der Veranstaltung abmelden")])->asImg(20) ?>
                     </a>