Deleted unused modal-whats-new component file

no issue

- backing class still existed for a component that previous had it's template removed

ghost/admin/app/components/modal-whats-new.js

@@ -1,13 +0,0 @@
-import ModalComponent from 'ghost-admin/components/modal-base';
-import {inject as service} from '@ember/service';
-
-export default ModalComponent.extend({
-    whatsNew: service(),
-
-    confirm() {},
-
-    actions: {
-        // noop - enter key shouldn't do anything
-        confirm() {}
-    }
-});