Changes

drop list of HNBAP UE Context

Last year, I have fixed some more ue_context leak situations.
But since we don't really use ue_context for anything, we could also
just drop this completely.

On HNBAP UE Register, we collect the ue_contexts in a ue_list. But we
never do anything with this list, at all. Furthermore, users are
reporting the list of ue_context growing indefinitely.

Simply drop the ue_context listing. Simply acknowledge all HNBAP UE
Register and DeRegister requests without storing any context IDs.

Change-Id: Ida7eadf36abcf465ae40003725c49e8e321a28c9