Limit req: removed check for unknown limit_req_zone.

With 48b3d5ddfb03, it's possible to specify limit_req_zone after limit_req.
diff --git a/src/http/modules/ngx_http_limit_req_module.c b/src/http/modules/ngx_http_limit_req_module.c
index 5d5cbe9..9059ac3 100644
--- a/src/http/modules/ngx_http_limit_req_module.c
+++ b/src/http/modules/ngx_http_limit_req_module.c
@@ -919,13 +919,6 @@
         return NGX_CONF_ERROR;
     }
 
-    if (shm_zone->data == NULL) {
-        ngx_conf_log_error(NGX_LOG_EMERG, cf, 0,
-                           "unknown limit_req_zone \"%V\"",
-                           &shm_zone->shm.name);
-        return NGX_CONF_ERROR;
-    }
-
     limits = lrcf->limits.elts;
 
     if (limits == NULL) {