Disable FAFTSequence._modify_usb_kernel() until it can be made safe.

The method attempts to overwrite local USB storage on the servod
device; however, as written it can't be guaranteed not to overwrite
a block device for critical internal disks on the autotest server.

BUG=chromium-os:38580
TEST=None

Change-Id: I268fa917b9adb93b8571ef0a7af5a16b26f9ff81
Reviewed-on: https://gerrit.chromium.org/gerrit/42589
Commit-Queue: Richard Barnette <jrbarnette@chromium.org>
Tested-by: Richard Barnette <jrbarnette@chromium.org>
Reviewed-by: Richard Barnette <jrbarnette@chromium.org>
Reviewed-by: Vadim Bendebury <vbendeb@chromium.org>
1 file changed