cluster: migrate round_robin_handle to internal assert

Use smaller internal assert module for round_robin_handle.js.

Author: Trott

lib/internal/cluster/round_robin_handle.js

@@ -1,5 +1,5 @@
 'use strict';
-const assert = require('assert');
+const assert = require('internal/assert');
 const net = require('net');
 const { sendHelper } = require('internal/cluster/utils');
 const uv = internalBinding('uv');