hnbgw: TC_rab_assign_mgcp_to: Allow hnbgw to tear down CN conn at SCCP level
Newer versions of osmo-hnbgw will be tearing down the CN conn at SCCP level by transmitting an RLSD in this scenario, instead of sending a Iu RANAP Release Request crafted by the osmo-hnbgw itself. The HNBGW is expected to forward RANAP messages between UE/HNB and CN, not to generate them.
