CLEANUP: init: remove useless assignment of nbthread

The old test consisting in setting global.nbthread if lower than 1
is useless nowadays since it's already done in check_config_validity().
diff --git a/src/haproxy.c b/src/haproxy.c
index 7e67f00..178f274 100644
--- a/src/haproxy.c
+++ b/src/haproxy.c
@@ -2227,6 +2227,7 @@
 		exit(1);
 	}
 
+	/* Note: global.nbthread will be initialized as part of this call */
 	err_code |= check_config_validity();
 	for (px = proxies_list; px; px = px->next) {
 		struct server *srv;
@@ -2598,9 +2599,6 @@
 		global.mode &= ~(MODE_DAEMON | MODE_QUIET);
 	}
 
-	if (global.nbthread < 1)
-		global.nbthread = 1;
-
 	/* Realloc trash buffers because global.tune.bufsize may have changed */
 	if (!init_trash_buffers(0)) {
 		ha_alert("failed to initialize trash buffers.\n");