cmd/go: remove inadvertent comment for vgo

This change was introduced while adding the dark copy
of golang.org/x/vgo in CL 118095.

While the comment made sense in a separate vgo repo, when it is
merged with the main repo, this should not remain.

Found while running mkalldocs.sh in CL 119695.

Change-Id: I112a4629c415032bd29e165ac1c27a0f3cabeede
Reviewed-on: https://go-review.googlesource.com/119938Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/go/internal/help/help.go

@@ -64,10 +64,6 @@ func Help(args []string) {
 
 var usageTemplate = `Go is a tool for managing Go source code.
 
-This is vgo, an experimental go command with support for package versioning.
-Even though you are invoking it as vgo, most of the messages printed will
-still say "go", not "vgo". Sorry.
-
 Usage:
 
 	go command [arguments]