vty: remove ignored logging parameters

Since ce9fec3e896571835ac5bfd2980d6836f2b29f0d libosmocore ignores
parameters to log_vty_command_* functions. Hence parameter of
logging_vty_add_cmds() is ignored too. As we depend on much later
libosmocore version anyway, we can simplify code somewhat by removing
parameters which will be ignored anyway.

Change-Id: I62f752fd88f1d8fefa563648f9864c7c31f87991
diff --git a/openbsc/src/gprs/sgsn_main.c b/openbsc/src/gprs/sgsn_main.c
index b4be631..af5cced 100644
--- a/openbsc/src/gprs/sgsn_main.c
+++ b/openbsc/src/gprs/sgsn_main.c
@@ -340,7 +340,7 @@
 
 	vty_info.copyright = openbsc_copyright;
 	vty_init(&vty_info);
-	logging_vty_add_cmds(&gprs_log_info);
+	logging_vty_add_cmds(NULL);
 	osmo_stats_vty_add_cmds(&gprs_log_info);
 	sgsn_vty_init();
 	ctrl_vty_init(tall_bsc_ctx);