This is an enhancement for the podman system prune feature.
In this issue, it is mentioned that 'network prune' should be
wired into 'podman system prune'
https://github.com/containers/podman/issues/8673
Therefore, I add the function to remove unused networks.
Signed-off-by: Toshiki Sonoda <sonoda.toshiki@fujitsu.com>