http: remove dualstack since its deprecated (#16940)

Deprecated: Fast Fallback is enabled by default. To
disable, set FallbackDelay to a negative value.

https://golang.org/src/net/dial.go

pkg/api/grafana_com_proxy.go

@@ -17,7 +17,6 @@ var grafanaComProxyTransport = &http.Transport{
 	Dial: (&net.Dialer{
 		Timeout:   30 * time.Second,
 		KeepAlive: 30 * time.Second,
-		DualStack: true,
 	}).Dial,
 	TLSHandshakeTimeout: 10 * time.Second,
 }