on HNBAP HNB Register, clear previous UE state if any
when a HNB registers with osmo-hnbgw, and the same HNB is already registered, osmo-hnbgw keeps the HNB state and simply logs "(duplicated)" with the HNB Register Request log.
A HNB should only register when it has restarted, and no UE state should stay around when that happens.
So, upon HNB Register Request, if we find that HNB already connected, clear out any previous state on that HNB.
Otherwise we likely keep stale UE state and stale SCCP conns indefinitely, by ignoring a restart of a HNB.