chore(js): remove Array.prototype.flat polyfill

Array.prototype.flat has been supported natively since Chrome 69,
Firefox 62, Safari 12 (2018), and Edge 79. The polyfill was originally
added for iOS < 12 support, which is no longer a target for Flarum 2.x.

Removing it saves ~8.5 KiB from the bundled forum.js output.

Author: imorland

framework/core/js/dist/admin.js

@@ -25,8 +25,6 @@ import app from './app';
 
 export { app };
 
-import './utils/arrayFlatPolyfill';
-
 const tooltipGen = $.fn.tooltip;
 
 // Remove in a future version of Flarum.