Revert "update_engine: Temporarily disable XZ generation"

This reverts commit cb2da0df.

Reason for revert: <paygens are running in chroot now sow lzma dependency should be fixed by now>

BUG=chromium:806613
TEST=unittest

Original change's description:
> update_engine: Temporarily disable XZ generation
>
> Disable generating XZ operations until crbug.com/806613 is fixed.
>
> BUG=chromium:806613
> TEST=unit tests pass
>
> Change-Id: I64b9852b447f7ff1f1a45f10205e3b23a2591cca
> Reviewed-on: https://chromium-review.googlesource.com/890384


> Commit-Ready: Amin Hassani <ahassani@chromium.org>
> Tested-by: Amin Hassani <ahassani@chromium.org>
> Reviewed-by: Amin Hassani <ahassani@chromium.org>

TBR=benchan@chromium.org,senj@chromium.org,ahassani@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: chromium:806613
Change-Id: Ic918b7e4febf284f38b2ca3119c1fcbf57538a89
Reviewed-on: https://chromium-review.googlesource.com/952378


Commit-Ready: Amin Hassani <ahassani@chromium.org>
Tested-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Amin Hassani <ahassani@chromium.org>