Enforce the removal of signatures in `podman save` to restore behavior
prior to the migration to libimage. We may consider improving on that
in the future. For details, please refer to the excellent summary by
@mtrmac [1].
[NO TESTS NEEDED] - manually verified but exisiting tests need some
further investigation (see [1]).
[1] https://github.com/containers/podman/pull/11669#issuecomment-925250264
Signed-off-by: Valentin Rothberg <rothberg@redhat.com>