Lower log level when no connection (#14800)

* Lower log level when no connection

* Fix style

Author: AlbumenJ

dubbo-remoting/dubbo-remoting-api/src/main/java/org/apache/dubbo/remoting/transport/AbstractServer.java

@@ -198,10 +198,7 @@ public void connected(Channel ch) throws RemotingException {
     @Override
     public void disconnected(Channel ch) throws RemotingException {
         if (getChannelsSize() == 0) {
-            logger.warn(
-                    INTERNAL_ERROR,
-                    "unknown error in remoting module",
-                    "",
+            logger.info(
                     "All clients has disconnected from " + ch.getLocalAddress() + ". You can graceful shutdown now.");
         }
         super.disconnected(ch);