runtime: remove unused copies of special stack guards

There are two copies each of the stackPreempt/_StackPreempt and
stackFork/_StackFork constants. Remove the ones left over from C that
are no longer used.

Change-Id: I849604c72c11e4a0cb08e45e9817eb3f5a6ce8ba
Reviewed-on: https://go-review.googlesource.com/43638Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/stack.go

@@ -102,15 +102,6 @@ const (
 	_StackLimit = _StackGuard - _StackSystem - _StackSmall
 )
 
-// Goroutine preemption request.
-// Stored into g->stackguard0 to cause split stack check failure.
-// Must be greater than any real sp.
-// 0xfffffade in hex.
-const (
-	_StackPreempt = uintptrMask & -1314
-	_StackFork    = uintptrMask & -1234
-)
-
 const (
 	// stackDebug == 0: no logging
 	//            == 1: logging of per-stack operations