vendor: Revert to github.com/jackpal/gateway instead of fork

It now includes all the fixes

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3246

vendor/github.com/jackpal/gateway/gateway_darwin.go

@@ -0,0 +1,16 @@
+package gateway
+
+import (
+	"net"
+	"os/exec"
+)
+
+func DiscoverGateway() (net.IP, error) {
+	routeCmd := exec.Command("/sbin/route", "-n", "get", "0.0.0.0")
+	output, err := routeCmd.CombinedOutput()
+	if err != nil {
+		return nil, err
+	}
+
+	return parseDarwinRouteGet(output)
+}