http2: remove references to defunct http2.golang.org test server

The http2 demo server previously served at http2.golang.org has been
stopped (see golang/go#49301). This change removes a link to it from
the top-level package docs (which now just redirects to go.dev), and
switches the default hostname for the TransportExternal test to
go.dev.

It also delete an unused transport_test flag.

Updates golang/go#49301
Fixes golang/go#51540

Change-Id: I375250a8fec087124d42ed5f1086986e73e7bdfc
Reviewed-on: https://go-review.googlesource.com/c/net/+/693895
Reviewed-by: Mark Freeman <markfreeman@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

http2/http2.go

@@ -11,8 +11,6 @@
 // requires Go 1.6 or later)
 //
 // See https://http2.github.io/ for more information on HTTP/2.
-//
-// See https://http2.golang.org/ for a test server running this code.
 package http2 // import "golang.org/x/net/http2"
 
 import (

http2/transport_test.go

@@ -40,8 +40,7 @@ import (
 
 var (
 	extNet        = flag.Bool("extnet", false, "do external network tests")
-	transportHost = flag.String("transporthost", "http2.golang.org", "hostname to use for TestTransport")
-	insecure      = flag.Bool("insecure", false, "insecure TLS dials") // TODO: dead code. remove?
+	transportHost = flag.String("transporthost", "go.dev", "hostname to use for TestTransport")
 )
 
 var tlsConfigInsecure = &tls.Config{InsecureSkipVerify: true}