code: func (s *IGDService) DeletePortMapping(protocol nat.Protocol, externalPort int) error {
  	tpl := `<u:DeletePortMapping xmlns:u="%s">
  	<NewRemoteHost></NewRemoteHost>
  	<NewExternalPort>%d</NewExternalPort>
  	<NewProtocol>%s</NewProtocol>
  	</u:DeletePortMapping>`
  	body := fmt.Sprintf(tpl, s.URN, externalPort, protocol)
  
  	_, err := soapRequest(s.URL, s.URN, "DeletePortMapping", body)
  	return err
  }
docs: // DeletePortMapping deletes a port mapping from the specified IGD service.