cmd/bundle: drop old +build lines

The +build to go:build syntax conversion has progressed enough that
it's fine to write only the new syntax now. All supported (and many
unsupported) Go releases understand it.

For golang/go#41184.
For golang/go#60268.

Change-Id: I8c1600577a21f4c7c89123302ca976d881a69841
Reviewed-on: https://go-review.googlesource.com/c/tools/+/536075
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

cmd/bundle/main.go

@@ -228,7 +228,6 @@ func bundle(src, dst, dstpkg, prefix, buildTags string) ([]byte, error) {
 	var out bytes.Buffer
 	if buildTags != "" {
 		fmt.Fprintf(&out, "//go:build %s\n", buildTags)
-		fmt.Fprintf(&out, "// +build %s\n\n", buildTags)
 	}
 
 	fmt.Fprintf(&out, "// Code generated by golang.org/x/tools/cmd/bundle. DO NOT EDIT.\n")

cmd/bundle/testdata/out.golden

@@ -1,5 +1,4 @@
 //go:build tag
-// +build tag
 
 // Code generated by golang.org/x/tools/cmd/bundle. DO NOT EDIT.
 //   $ bundle