Reorder DLSCH MAC PDU logic

Before this commit, the DLSCH scheduler would construct the MAC PDU by
reading RLC data into a memory on the stack, and then construct the PDU
with CEs first. There are at least two problems:
- we need to keep track of the exact number of bytes of CEs (cumbersome)
  to calculate the number of MAC SDUs to include
- we needlessly copy data around.

This commit does the following instead:
- write all CEs first (no need of keeping track of this in DLSCH and a
  separate function)
- then read MAC SDUs directly into nFAPI as much as possible or
  necessary, without recopying

openair2/LAYER2/NR_MAC_gNB/mac_proto.h

@@ -64,17 +64,6 @@ void clear_nr_nfapi_information(gNB_MAC_INST * gNB,
 void gNB_dlsch_ulsch_scheduler(module_id_t module_idP,
 			       frame_t frame_rxP, sub_frame_t slot_rxP);
 
-int nr_generate_dlsch_pdu(module_id_t Mod_idP,
-                          NR_UE_sched_ctrl_t *ue_sched_ctl,
-                          unsigned char *sdus_payload,
-                          unsigned char *mac_pdu,
-                          unsigned char num_sdus,
-                          unsigned short *sdu_lengths,
-                          unsigned char *sdu_lcids,
-                          unsigned char drx_cmd,
-                          unsigned char *ue_cont_res_id,
-                          unsigned short post_padding);
-
 void nr_schedule_ue_spec(module_id_t module_id,
                          frame_t frame,
                          sub_frame_t slot);