[release-branch.go1.13] http2/h2demo: remove h2demo build constraint

The build constraint is no longer useful. It doesn't prevent this
package contributing module requirements to x/net, that was already
resolved by carving h2demo into its own module in golang/go#30685.
Few people do go get -u golang.org/x/net/... in GOPATH mode by now,
so there's no need to optimize for avoiding polluting GOPATH/bin.

Removing the build constraint allows the package to be visible and
tested by trybots and builders. It's also simpler.

Updates golang/go#34361

Change-Id: I84b5d70aab210ca8e4f5494160ae4d9049ef08ad
Reviewed-on: https://go-review.googlesource.com/c/net/+/196036
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
(cherry picked from commit a8b05e9114)
Reviewed-on: https://go-review.googlesource.com/c/net/+/198917

http2/h2demo/Dockerfile

@@ -25,7 +25,7 @@ COPY . /go/src/golang.org/x/net
 
 # Install binary to /go/bin:
 WORKDIR /go/src/golang.org/x/net/http2/h2demo
-RUN go install -tags "h2demo netgo"
+RUN go install -tags "netgo"
 
 
 FROM debian:stretch

http2/h2demo/h2demo.go

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build h2demo
-
 package main
 
 import (

http2/h2demo/tmpl.go

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build h2demo
-
 package main
 
 import "html/template"