Windows doesn't remove an open file.

Fixes #17345

Change-Id: I283930c7d6eb3bc09ad208191afefe989804ce32
Reviewed-on: https://go-review.googlesource.com/30430
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>