futility: Drop futility execution logging to /tmp/futility.log

This functionality is not used anymore and is also a potential security
vulnerability.

BUG=None
BRANCH=None
TEST=sudo FEATURES=test emerge vboot_reference

Change-Id: Ie1bd38a26faa240f8cd00ab7717ed58489223e31
Signed-off-by: Jakub Czapiga <czapiga@google.com>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/5972390
Reviewed-by: Julius Werner <jwerner@chromium.org>