Remove umount check. - address Frysinger comments in pervious commit. - umount check will be done at mount_gpt_image.sh. BUG=b:62703887 TEST=Manually Change-Id: I548a54533ca55a7c47b31cb4f3df32d8f517c938 Reviewed-on: https://chromium-review.googlesource.com/544902 Commit-Ready: danny chan <dchan@chromium.org> Tested-by: danny chan <dchan@chromium.org> Reviewed-by: Mike Frysinger <vapier@chromium.org>