runtime: remove syscall.NewCallbackCDecl on Windows

It is not possible to use (there is no declaration in package syscall),
and no one seems to care.

Alex Brainman may bring this back properly for Go 1.3.

Fixes #6338.

R=golang-dev, r, alex.brainman
CC=golang-dev
https://golang.org/cl/14287043

src/pkg/runtime/syscall_windows.goc

@@ -40,9 +40,14 @@ func NewCallback(fn Eface) (code uintptr) {
 	code = (uintptr)runtime·compilecallback(fn, true);
 }
 
+/*
+ * If this is needed, uncomment here and add a declaration in package syscall
+ * next to the NewCallback declaration.
+ *
 func NewCallbackCDecl(fn Eface) (code uintptr) {
 	code = (uintptr)runtime·compilecallback(fn, false);
 }
+ */
 
 func Syscall(fn uintptr, nargs uintptr, a1 uintptr, a2 uintptr, a3 uintptr) (r1 uintptr, r2 uintptr, err uintptr) {
 	WinCall c;