Cleanup handover required transfer IE

Previously, the RRC layer was incorrectly allocating stack memory for the ho_required_transfer field and passing it to NGAP, however it was not used. The IE is mandatory but contains only 1 optional IE, therefore is encoded in the NG message with empty content.

Changes:
- Remove unnecessary ho_required_transfer field from pdusession_resource_t definition, the IE is encoded anyway with empty content. If Direct Forwarding Path Availability is needed at any stage in the future it can be added to pdusession_resource_t and filled in RRC.
- Add clarifying comment about empty HandoverRequiredTransfer structure

openair2/COMMON/ngap_messages_types.h

@@ -545,8 +545,6 @@ typedef enum {
 typedef struct {
   // PDU Session ID
   uint8_t pdusession_id;
-  // Handover Required Transfer
-  byte_array_t ho_required_transfer;
 } pdusession_resource_t;
 
 /* 3GPP TS 38.413 9.2.3.1 */