After support for the sheepdog storage driver backend was removed we
don't need to install it any longer in the containers.
Regenerate the dockerfiles after:
https://gitlab.com/libvirt/libvirt-ci/-/merge_requests/314
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Jiri Denemark <jdenemar@redhat.com>