cmd/go/testdata/mod: remove unused research.swtch.com/vgo-tour

The test that used that module was removed in
https://golang.org/cl/128900.

Change-Id: Id96270a52398c8ccc09821efb2a6a6b4764f44d9
Reviewed-on: https://go-review.googlesource.com/c/151560
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/go/testdata/mod/research.swtch.com_vgo-tour_v1.0.0.txt

-research.swtch.com/vgo-tour@v1.0.0
-
--- .mod --
-module "research.swtch.com/vgo-tour"
--- .info --
-{"Version":"v1.0.0","Name":"84de74b35823c1e49634f2262f1a58cfc951ebae","Short":"84de74b35823","Time":"2018-02-20T00:04:00Z"}
--- go.mod --
-module "research.swtch.com/vgo-tour"
--- hello.go --
-// Copyright 2018 The Go Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-package main
-
-import (
-	"fmt"
-	"rsc.io/quote"
-)
-
-func main() {
-	fmt.Println(quote.Hello())
-}