windows: remove unused errString type

It's no longer used since CL 165759.

Change-Id: Ie8c834a6dd1147889ec47bf92a5d4cce08bbf4fd
GitHub-Last-Rev: 3f0c460
GitHub-Pull-Request: #241
Reviewed-on: https://go-review.googlesource.com/c/sys/+/638716
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Jorropo <jorropo.pgm@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Jorropo <jorropo.pgm@gmail.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>

windows/dll_windows.go

@@ -410,7 +410,3 @@ func loadLibraryEx(name string, system bool) (*DLL, error) {
 	}
 	return &DLL{Name: name, Handle: h}, nil
 }
-
-type errString string
-
-func (s errString) Error() string { return string(s) }