Replace NGAP_MAX_PDU_SESSION and E1AP_MAX_NUM_PDU_SESSIONS with
shared NR_MAX_NB_PDU_SESSIONS constant. The value (256) is consistent
with both TS 38.331, TS 38.413 and TS 38.463
All protocol layers (NGAP, E1AP, RRC, SDAP) now use a single
shared constant for maximum PDU sessions per UE, ensuring
consistency across the codebase
E1AP_MAX_NUM_PDU_SESSIONS (was 4) is removed from e1ap_messages_types.h
Signed-off-by: Guido Casati <guido.casati@openairinterface.org>