runtime: delete unnecessary confusing code

The code in GC that handles gp->gobuf.ctxt is wrong,
because it does not mark the ctxt object itself,
if just queues the ctxt object for scanning.
So the ctxt object can be collected as garbage.
However, Gobuf.ctxt is void*, so it's always marked and
scanned through G.

LGTM=khr
R=golang-codereviews, khr
CC=golang-codereviews, khr, rsc
https://golang.org/cl/105490044

src/pkg/runtime/mgc0.c

@@ -1615,8 +1615,6 @@ addstackroots(G *gp, Workbuf **wbufp)
 	int32 n;
 	Stktop *stk;
 	uintptr sp, guard;
-	void *base;
-	uintptr size;
 
 	switch(gp->status){
 	default:
@@ -1652,9 +1650,6 @@ addstackroots(G *gp, Workbuf **wbufp)
 		sp = gp->sched.sp;
 		stk = (Stktop*)gp->stackbase;
 		guard = gp->stackguard;
-		// For function about to start, context argument is a root too.
-		if(gp->sched.ctxt != 0 && runtime·mlookup(gp->sched.ctxt, &base, &size, nil))
-			enqueue1(wbufp, (Obj){base, size, 0});
 	}
 	if(ScanStackByFrames) {
 		USED(sp);

src/pkg/runtime/runtime.h

@@ -213,7 +213,7 @@ struct	Gobuf
 	uintptr	sp;
 	uintptr	pc;
 	G*	g;
-	void*	ctxt;
+	void*	ctxt; // this has to be a pointer so that GC scans it
 	uintreg	ret;
 	uintptr	lr;
 };