os: delete unused errClosed

os: delete unused errClosed

errClosed was introduced by CL 163058 and was supposed to be removed by CL 243906,
but somehow it was left out, now we should get it deleted.

Change-Id: I74c4b36b8bbc4844e1860acb022a16b0aa3272b2
Reviewed-on: https://go-review.googlesource.com/c/go/+/427035
Run-TryBot: Andy Pan <panjf2000@gmail.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>