Remove allsettled polyfill

app/index.js

@@ -5,26 +5,6 @@
 
 import { PageConfig, URLExt } from '@jupyterlab/coreutils';
 
-// Promise.allSettled polyfill, until our supported browsers implement it
-// See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/allSettled
-if (Promise.allSettled === undefined) {
-  Promise.allSettled = promises =>
-    Promise.all(
-      promises.map(promise =>
-        promise.then(
-          value => ({
-            status: 'fulfilled',
-            value
-          }),
-          reason => ({
-            status: 'rejected',
-            reason
-          })
-        )
-      )
-    );
-}
-
 require('./style.js');
 require('./extraStyle.js');