Revert "DEV: Fix a deprecated ember import (#9903)"

This reverts commit 75f46ca632.

This unfortunately breaks

`RAILS_ENV=production bin/rake assets:precompile`

I don't think discourse-loader.js is transpiled

app/assets/javascripts/discourse-loader.js

@@ -29,7 +29,6 @@ var define, requirejs;
         inject: Ember.inject.controller
       },
       "@ember/debug": {
-        isTesting: () => Ember.testing,
         warn: Ember.warn
       },
       "@ember/object": {

app/assets/javascripts/discourse/app/helpers/page-reloader.js

@@ -1,9 +1,9 @@
 import EmberObject from "@ember/object";
-import { isTesting } from "@ember/debug";
+import Ember from "ember";
 
 export default EmberObject.create({
-  reload() {
-    if (!isTesting()) {
+  reload: function() {
+    if (!Ember.testing) {
       location.reload();
     }
   }