runtime: eliminate unused _GCstw phase

Change-Id: Ie94cd17e1975fdaaa418fa6a7b2d3b164fedc135
Reviewed-on: https://go-review.googlesource.com/16057Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/mgc.go

@@ -236,7 +236,6 @@ var gcBlackenPromptly bool
 
 const (
 	_GCoff             = iota // GC not running; sweeping in background, write barrier disabled
-	_GCstw                    // unused state
 	_GCscan                   // GC collecting roots into workbufs, write barrier ENABLED
 	_GCmark                   // GC marking from workbufs, write barrier ENABLED
 	_GCmarktermination        // GC mark termination: allocate black, P's help GC, write barrier ENABLED