4G RLC/PDCP interface: remove function pointers

Remove function pointers that were used in the past to implement the
F1(-U) interface in 4G, i.e., to have different functions to handle
split/monolithic operations. As we don't have F1 in 4G anymore (see
commit 9dc2282249), we don't need the
function pointers anymore.

On that occasion, rename the 5G functions by prepending nr_ to them;
this allows to refactor the interface in 5G further (which is not the
focus of this changeset).

executables/lte-softmodem.c

@@ -416,9 +416,6 @@ static void init_pdcp(void)
   pdcp_initmask = pdcp_initmask | ENB_NAS_USE_TUN_W_MBMS_BIT;
 
   pdcp_module_init(pdcp_initmask, 0);
-
-  pdcp_set_rlc_data_req_func(rlc_data_req);
-  pdcp_set_pdcp_data_ind_func(pdcp_data_ind);
 }
 
 static  void wait_nfapi_init(char *thread_name) {