Removed old unused default configuration option parent:text from sonata_type_model_list, refs #2455

Form/Type/ModelTypeList.php

@@ -61,7 +61,6 @@ class ModelTypeList extends AbstractType
         $resolver->setDefaults(array(
             'model_manager'     => null,
             'class'             => null,
-            'parent'            => 'text',
             'btn_add'           => 'link_add',
             'btn_list'          => 'link_list',
             'btn_delete'        => 'link_delete',