Remove --usim-test command line option

Remove the --usim-test option: to my knowledge, it's not used by users,
and is misleadingly placed in various places where there should not be a
difference w.r.t. USIM configuration.

When making the changes, I assumed that --usim-test would be 0 (the
default), which is the case in all executables when not specified in
options except for the nr_dlsim, nr_pbchsmi, nr_ulsim simulators.

The only exception is the initialization of PDCP from the MAC, which is
not correct (the PDCP is in the CU, so it does not make sense to call it
from the MAC which is in the DU). Instead, always initialize from main,
even in the NSA case (NSA was covered by the incorrect initialization
from MAC).

This in turn let to some more cleanup around function du_rlc_data_req().
After receiving GTP packets at the DU, they were given to the PDCP to
enqueue the packet at RLC. This does not work anymore, since the PDCP is
not initialized; it also does not seem necessary, as this enqueue
functionality is necessary to decouple PDCP and RLC to avoid deadlocks
in monolithic, but in split-mode, this cannot happen. Instead, call
directly into the RLC when receiving GTP packets.

executables/nr-softmodem.c

@@ -261,9 +261,9 @@ static int create_gNB_tasks(ngran_node_t node_type, configmodule_interface_t *cf
   RC.nrrrc = calloc(1, sizeof(*RC.nrrrc));
   RC.nrrrc[0] = RCconfig_NRRRC();
 
-  if (!get_softmodem_params()->nsa && !(node_type == ngran_gNB_DU)) {
+  if (node_type != ngran_gNB_DU) {
     // we start pdcp in both cuup (for drb) and cucp (for srb)
-    init_pdcp();
+    nr_pdcp_layer_init();
   }
 
   if (get_softmodem_params()->nsa) { //&& !NODE_IS_DU(node_type)
@@ -513,12 +513,6 @@ static  void wait_nfapi_init(char *thread_name)
   pthread_mutex_unlock(&nfapi_sync_mutex);
 }
 
-void init_pdcp(void) {
-  if (!NODE_IS_DU(get_node_type())) {
-    nr_pdcp_layer_init();
-  }
-}
-
 #ifdef E2_AGENT
 static void initialize_agent(ngran_node_t node_type, e2_agent_args_t oai_args)
 {