only remove the notifications for one course not for all, fixes #2024

Closes #2024

Merge request studip/studip!1316

lib/models/CourseMember.class.php

@@ -152,7 +152,10 @@ class CourseMember extends SimpleORMap implements PrivacyObject
 
     public function cbRemoveNotifications()
     {
-        CourseMemberNotification::deleteBySQL('user_id = ?', [$this->user_id]);
+        CourseMemberNotification::deleteBySQL(
+            'user_id = ? AND seminar_id = ?',
+            [$this->user_id, $this->seminar_id]
+        );
     }
 
     /**