nat/pmp.go
func (n *npmp) DeleteMapping(protocol string, extPort, intPort int) (err error) {
	_, err = n.c.AddPortMapping(strings.ToLower(protocol), intPort, 0, 0)
	return err
}
func (n *npmp) DeleteMapping(protocol string, extPort, intPort int) (err error) {
	_, err = n.c.AddPortMapping(strings.ToLower(protocol), intPort, 0, 0)
	return err
}
// DeleteMapping removes the port mapping to NAT-PMP interface.