lava: Drop tar unpack macro since aum tests no longer redeploy overlays

Now that the aum tests no longer redeploy the overlay at every reboot
there's no need to disable verbose reporting in those cases.

Since that was the entire purpose of the `tar_unpack_cmd()` macro and
the relative supporting code, let's drop it.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>