archive/zip: remove unused File.descErr field

archive/zip: remove unused File.descErr field

Found via staticcheck. Unused as of CL 357489.

Change-Id: I3aa409994ba4388912ac7e7809168529a5b6e31c
Reviewed-on: https://go-review.googlesource.com/c/go/+/403814
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Kale B <kale@lemnisys.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: David Chase <drchase@google.com>