squash: unsquash the root image instead of mounting it on shutdown

squash: unsquash the root image instead of mounting it on shutdown

When building squash image, squash module forgot to install the new
shutdown.sh, and the shutdown hooks are always skipped on ordinary
shutdown if squash module is enabled.

The new shutdown.sh will remount the squash image and then everything
will just work, but currently re-mounting the squash image on shutdown
may have selinux problem and make the system hang, and there is no
easy way to fix it.

So skip fixing the shutdown.sh not being install problem, instead
just drop the new shutdown.sh, and unsquash the image on ordinary
shutdown, which is safer and should always work.

Signed-off-by: Kairui Song <kasong@redhat.com>