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>