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.

openair2/X2AP/x2ap_eNB.h

@@ -46,6 +46,8 @@ int x2ap_eNB_init_sctp (x2ap_eNB_instance_t *instance_p,
 
 void *x2ap_task(void *arg);
 
+int is_x2ap_enabled(void);
+
 #endif /* X2AP_H_ */
 
 /**