erp5_core: get_fields_in_group already exclude disabled field
https://lab.nexedi.com/nexedi/erp5/blob/bb7d107124e96a6176f56226d3093d8845c3949d/product/Formulator/Form.py#L228
https://lab.nexedi.com/nexedi/erp5/blob/bb7d107124e96a6176f56226d3093d8845c3949d/product/Formulator/Form.py#L228