func (p *Pinger) RemoveIP(ipaddr string) error {
	addr := net.ParseIP(ipaddr)
	if addr == nil {
		return fmt.Errorf("%s is not a valid textual representation of an IP address", ipaddr)
	}
	p.mu.Lock()
	delete(p.addrs, addr.String())
	p.mu.Unlock()
	return nil
}
func (p *Pinger) RemoveIP(ipaddr string) error {
	addr := net.ParseIP(ipaddr)
	if addr == nil {
		return fmt.Errorf("%s is not a valid textual representation of an IP address", ipaddr)
	}
	p.mu.Lock()
	delete(p.addrs, addr.String())
	p.mu.Unlock()
	return nil
}
// RemoveIP removes an IP address from Pinger. ipaddr arg should be a string
// like "192.0.2.1".