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/models/datasource_cache.go

@@ -61,7 +61,6 @@ func (ds *DataSource) GetHttpTransport() (*http.Transport, error) {
 		Dial: (&net.Dialer{
 			Timeout:   time.Duration(setting.DataProxyTimeout) * time.Second,
 			KeepAlive: 30 * time.Second,
-			DualStack: true,
 		}).Dial,
 		TLSHandshakeTimeout:   10 * time.Second,
 		ExpectContinueTimeout: 1 * time.Second,