Removed trailing Admin model for Collections

ref https://linear.app/ghost/issue/ENG-1805/remove-collections-code

- this was still lingering around because I forgot to delete it in the
  initial commit

ghost/admin/app/models/collection.js

@@ -1,33 +0,0 @@
-import Model from '@ember-data/model';
-import ValidationEngine from 'ghost-admin/mixins/validation-engine';
-import {attr} from '@ember-data/model';
-import {computed} from '@ember/object';
-import {inject as service} from '@ember/service';
-
-export default Model.extend(ValidationEngine, {
-    validationType: 'collection',
-
-    title: attr('string'),
-    slug: attr('string'),
-    description: attr('string'),
-    type: attr('string', {defaultValue: 'manual'}),
-    filter: attr('string'),
-    featureImage: attr('string'),
-    createdAtUTC: attr('moment-utc'),
-    updatedAtUTC: attr('moment-utc'),
-    createdBy: attr('number'),
-    updatedBy: attr('number'),
-    count: attr('raw'),
-
-    posts: attr('raw'),
-
-    postIds: computed('posts', function () {
-        if (this.posts && this.posts.length) {
-            return this.posts.map(post => post.id);
-        } else {
-            return [];
-        }
-    }),
-
-    feature: service()
-});