Remove unnecessary fetch-setting

Originally this was failing because it only had a getter.
I tried replacing it with a computed property and that
succeeded, but since we have already stopped using
jQuery, we might as well remove it.

ui/app/adapters/application.js

@@ -10,10 +10,6 @@ export const namespace = 'v1';
 
 @classic
 export default class Application extends RESTAdapter {
-  // TODO: This can be removed once jquery-integration is turned off for
-  // the entire app.
-  useFetch = true;
-
   namespace = namespace;
 
   @service system;