windows: remove non-existing now mksyscall_windows.go -xsys flag

Updates golang/go#15167

Change-Id: I58bd7e4e6ba88a2ffba11506e5ac0495de303b5b
Reviewed-on: https://go-review.googlesource.com/23023
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>

windows/registry/syscall.go

@@ -8,7 +8,7 @@ package registry
 
 import "syscall"
 
-//go:generate go run $GOROOT/src/syscall/mksyscall_windows.go -xsys -output zsyscall_windows.go syscall.go
+//go:generate go run $GOROOT/src/syscall/mksyscall_windows.go -output zsyscall_windows.go syscall.go
 
 const (
 	_REG_OPTION_NON_VOLATILE = 0

windows/syscall_windows.go

@@ -14,7 +14,7 @@ import (
 	"unsafe"
 )
 
-//go:generate go run $GOROOT/src/syscall/mksyscall_windows.go -xsys -output zsyscall_windows.go eventlog.go service.go syscall_windows.go security_windows.go
+//go:generate go run $GOROOT/src/syscall/mksyscall_windows.go -output zsyscall_windows.go eventlog.go service.go syscall_windows.go security_windows.go
 
 type Handle uintptr