net/http: removing DualStack in DefaultTransport

Removing `DualStack` from `http.DefaultTransport` since it is deprecated. 
https://github.com/golang/go/blob/master/src/net/dial.go#L61

Author: ananyasaxena

src/net/http/transport.go

@@ -44,7 +44,6 @@ var DefaultTransport RoundTripper = &Transport{
 	DialContext: (&net.Dialer{
 		Timeout:   30 * time.Second,
 		KeepAlive: 30 * time.Second,
-		DualStack: true,
 	}).DialContext,
 	ForceAttemptHTTP2:     true,
 	MaxIdleConns:          100,