Revert "stateful_update: Deprecate/Delete"

This reverts commit 48c2e8a369c07d0b4ee1eaa2eba1e4ae13ee0123.

Reason for revert: autoupdater in Autotest is suspected to still need this, and this may be causing increased provision failures.  Revert since it's just removing an otherwise presumed unused file.

Original change's description:
> stateful_update: Deprecate/Delete
> 
> stateful_update has been chromitized. We don't need this script anymore.
> 
> BUG=chromium:1026933
> TEST=CQ passes
> 
> Cq-Depend: chromium:2054077
> Change-Id: Ibb707672a3d421bbc779c042afa9ddb143ef2efc
> Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/2054755
> Tested-by: Amin Hassani <ahassani@chromium.org>
> Commit-Queue: Amin Hassani <ahassani@chromium.org>
> Reviewed-by: Mike Frysinger <vapier@chromium.org>

Bug: chromium:1026933
Change-Id: If45b6812d35b4cd8f67ee9451013269cbc260d62
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/2133291
Reviewed-by: Allen Li <ayatane@chromium.org>
Reviewed-by: Amin Hassani <ahassani@chromium.org>
Commit-Queue: Allen Li <ayatane@chromium.org>
Tested-by: Allen Li <ayatane@chromium.org>