Revert "windows: make zsyscall_windows.go generatable on non-WSL env"

This reverts commit ad28b68e88.

Reason for revert: It completely breaks 'go generate' on windows

Update golang/go#32349

Change-Id: I4bedc6cf5130eb48c18c3208c8029433407430e4
Reviewed-on: https://go-review.googlesource.com/c/sys/+/179580
Reviewed-by: Takuto Ikuta <tikuta@google.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>

windows/mkerrors.go

@@ -0,0 +1,9 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build generate
+
+package windows
+
+//go:generate ./mkerrors.bash zerrors_windows.go