Changes

s1ap_proxy: remove eNB-ID from erab_uid()

This patch prepares for a follow-up change implementing handling of
the HANDOVER related PDUs.  The problem is that the HANDOVER REQUEST
PDU contains MME-UE-S1AP-ID, but not eNB-UE-S1AP-ID, while the E-RAB
registry requires both to be part of the erab_uid() tuple.

In the scope of an S1-connection, a pair of MME-UE-S1AP-ID and
E-RAB-ID values is enough to identify an E-RAB unambiguously,
thus we can safely remove eNB-UE-S1AP-ID from erab_uid().

Change-Id: I7277d19080795600252306dbfa0c733f996e026e