UserGroupBackend: Disable default backend type `ini'

We're not going to support this until a proper membership implementation
exists (or is required at all).

refs #8826

library/Icinga/Authentication/UserGroup/UserGroupBackend.php

@@ -21,7 +21,7 @@ class UserGroupBackend
      */
     protected static $defaultBackends = array(
         'db',
-        'ini'
+        //'ini'
     );
 
     /**