vty: cleanup logging functions

* remove unused parameter from logging_vty_add_cmds()
* mark log level descriptors static
* change internal static function int check_log_to_target() to more
  appropriate bool should_log_to_target()
* deprecate log_vty_command_*() from public API as it should only be
  used by logging_vty_add_cmds()

Change-Id: I0e9ddd7ba3ce211302d99a3494eb408907a2916e
Related: OS#71
diff --git a/tests/vty/vty_test.c b/tests/vty/vty_test.c
index 865c93e..1e1b495 100644
--- a/tests/vty/vty_test.c
+++ b/tests/vty/vty_test.c
@@ -318,7 +318,7 @@
 	vty_init(&vty_info);
 
 	/* Setup VTY commands */
-	logging_vty_add_cmds(&log_info);
+	logging_vty_add_cmds();
 	osmo_stats_vty_add_cmds();
 
 	test_cmd_string_from_valstr();