Merge branch 'ash2k/remove-deprecated-option' into 'master'

Remove deprecated option

See merge request gitlab-org/gitlab-workhorse!513

internal/upstream/roundtripper/transport.go

@@ -12,7 +12,6 @@ func newBackendTransport() (*http.Transport, *net.Dialer) {
 	dialler := &net.Dialer{
 		Timeout:   30 * time.Second,
 		KeepAlive: 30 * time.Second,
-		DualStack: true,
 	}
 
 	transport := &http.Transport{