refactor: avoid redeclaring const in xhr polling transport (#645)

Author: aeroxmotion

lib/transports/polling-xhr.js

@@ -15,7 +15,6 @@ const debug = require("debug")("engine.io-client:polling-xhr");
 function empty() {}
 
 const hasXHR2 = (function() {
-  const XMLHttpRequest = require("xmlhttprequest-ssl");
   const xhr = new XMLHttpRequest({ xdomain: false });
   return null != xhr.responseType;
 })();