windows: remove unused errString type

It's no longer used since CL 165759.

Change-Id: Ie8c834a6dd1147889ec47bf92a5d4cce08bbf4fd
GitHub-Last-Rev: 3f0c460db144b70b008a762f12a6e0c1ff6bdfbf
GitHub-Pull-Request: golang/sys#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>