Remove dummy app type declaration

Author: charlesfries

types/dummy/app.d.ts

@@ -1,8 +1,2 @@
 import 'ember-source/types';
 import 'ember-source/types/preview';
-
-declare module 'dummy/app' {
-  import Application from '@ember/application';
-
-  export default class App extends Application {}
-}