code: func ClearRetainedMessage(config *Config, topic string, timeout time.Duration) error {
  	return PublishMessage(config, &packet.Message{
  		Topic:   topic,
  		Payload: nil,
  		QOS:     0,
  		Retain:  true,
  	}, timeout)
  }
docs: // ClearRetainedMessage will connect to the specified broker and send an empty
  // retained message to force any already retained message to be cleared.