Reverting default X-Requested-With header for xhr

for more details see https://remysharp.com/2011/04/21/getting-cors-working

lib/adapters/xhr.js

@@ -25,9 +25,6 @@ module.exports = function xhrAdapter(resolve, reject, config) {
     config.headers || {}
   );
 
-  // Indicate that request is being made with XHR
-  requestHeaders['X-Requested-With'] = 'XMLHttpRequest';
-
   if (utils.isFormData(data)) {
     delete requestHeaders['Content-Type']; // Let the browser set it
   }