logging: Remove unused variable after recent changes

The variable is not used anymore as the decision is done in the
newly added check_log_to_target method.
diff --git a/src/logging.c b/src/logging.c
index 1c9c663..35746e9 100644
--- a/src/logging.c
+++ b/src/logging.c
@@ -361,7 +361,6 @@
 	subsys = map_subsys(subsys);
 
 	llist_for_each_entry(tar, &osmo_log_target_list, entry) {
-		int output = 0;
 		va_list bp;
 
 		if (!check_log_to_target(tar, subsys, level))