nwfilter: remove obsolete code related to firewalld
    
    There is a bunch of left over code in the nwfilter driver related to
    monitoring firewalld over dbus, that is no longer used since the
    conversion to use virFirewall APIs.
    
    Reviewed-by: Jiri Denemark <jdenemar@redhat.com>
    Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>