Refactored billing controller to Octane patterns

refs https://github.com/TryGhost/Ghost/issues/14101

- migrated to full native class syntax

ghost/admin/app/controllers/billing.js

@@ -1,12 +1,10 @@
 import Controller from '@ember/controller';
-import classic from 'ember-classic-decorator';
-import {alias} from '@ember/object/computed';
 
-@classic
 export default class BillingController extends Controller {
     queryParams = ['action'];
     action = null;
 
-    @alias('model')
-        guid;
+    get guid() {
+        return this.model;
+    }
 }