IniUserGroupBackend: Do not sort by parent when sorting by group_name

refs #8826

library/Icinga/Authentication/UserGroup/IniUserGroupBackend.php

@@ -35,20 +35,6 @@ class IniUserGroupBackend extends IniRepository implements UserGroupBackendInter
      */
     protected $filterColumns = array('group', 'parent');
 
-    /**
-     * The default sort rules to be applied on a query
-     *
-     * @var array
-     */
-    protected $sortRules = array(
-        'group_name' => array(
-            'columns'   => array(
-                'group_name',
-                'parent_name'
-            )
-        )
-    );
-
     /**
      * The value conversion rules to apply on a query
      *