Calling 'rmdir --parents /var/cache/fwupdate' will cause it to attempt
to rmdir /var/cache and /var. Those directories are very unlikely to be
empty, so it should always quietly fail. However, there's not benefit
in attempting those removals, so let's quit doing it.
It's possible that someone has removed fwupdate package prior to the
fwupd transition meaning that they might have some artifacts left
behind from fwupdate packaging. Clean up these artifacts.
This commit can be reverted after both Debian bullseye and Ubuntu
focal have been released.
