cmd/go: remove unused global var isGoRelease

This variable is not used anymore. It was used in the
TestPackageNotStaleWithTrailingSlash test.

Change-Id: I5e52d4d1d91592dd21e2d9cff96974a49d07f5f8
Reviewed-on: https://go-review.googlesource.com/126376
Run-TryBot: Yury Smolsky <yury@smolsky.by>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/go/go_test.go

@@ -1464,12 +1464,6 @@ func TestPackageMainTestCompilerFlags(t *testing.T) {
 	tg.grepStderr(`([\\/]compile|gccgo).* (-p p1|-fgo-pkgpath=p1).*p1\.go`, "should have run compile -p p1 p1.go")
 }
 
-// The runtime version string takes one of two forms:
-// "go1.X[.Y]" for Go releases, and "devel +hash" at tip.
-// Determine whether we are in a released copy by
-// inspecting the version.
-var isGoRelease = strings.HasPrefix(runtime.Version(), "go1")
-
 // Issue 12690
 func TestPackageNotStaleWithTrailingSlash(t *testing.T) {
 	skipIfGccgo(t, "gccgo does not have GOROOT")