Remove the os.Exit(0) to honor the deferred closing of the file.

Change-Id: Iaa9304d8203c8fec0ec728af669a94eadd36905c
Reviewed-on: https://go-review.googlesource.com/118915Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>