Changes

s1gw: correct handling of UE CONTEXT RELEASE REQUEST

By sending a UE CONTEXT RELEASE REQUEST, the eNB is requesting the
MME to terminate the UE context.  This procedure itself does not
terminate the UE context nor the associated E-RABs.  Therefore, the
S1GW should not terminate E-RAB FSMs upon receiving this PDU.

This means that we should not be expecting PFCP sesstion(s)
to be deleted in S1GW_ConnHdlr.f_ConnHdlr_ue_ctx_release_req().

Change-Id: Ibbb6da1f2888995608b980e7479758000e94864e
Related: osmo-s1gw.git If467162e871b7989f0578be30c04d1c32c263c99
Related: SYS#7738, SYS#7599