MINOR: listener: move the nice field to the bind_conf

This is another bind line setting which can move to the bind_conf.
Note that it leaves a 2-byte hole in the listener struct.
diff --git a/src/listener.c b/src/listener.c
index 86cf85c..1635576 100644
--- a/src/listener.c
+++ b/src/listener.c
@@ -1742,7 +1742,6 @@
 /* parse the "nice" bind keyword */
 static int bind_parse_nice(char **args, int cur_arg, struct proxy *px, struct bind_conf *conf, char **err)
 {
-	struct listener *l;
 	int val;
 
 	if (!*args[cur_arg + 1]) {
@@ -1756,9 +1755,7 @@
 		return ERR_ALERT | ERR_FATAL;
 	}
 
-	list_for_each_entry(l, &conf->listeners, by_bind)
-		l->nice = val;
-
+	conf->nice = val;
 	return 0;
 }