x2ap: timers, cleanup

This commit introduces X2AP timers (t_reloc_prep, tx2_reloc_overall).
You need to set the values in the configuration file.

X2AP can be enabled or disabled in the configuration file too (disabled
by default).

Some deadcode was removed.

cmake_targets/autotests/v2/config/enb.band38.tm1.usrpb210.tdd.10MHz.conf

@@ -141,6 +141,11 @@ eNBs =
                             }
                           );
 
+    ///X2
+    enable_x2 = "no";
+    t_reloc_prep      = 1000;      /* unit: millisecond */
+    tx2_reloc_overall = 2000;      /* unit: millisecond */
+
     NETWORK_INTERFACES :
     {
         ENB_INTERFACE_NAME_FOR_S1_MME            = "eth6";