Clean up asset manager

src/asset_manager/model/Asset.js

@@ -1,6 +1,4 @@
-var Backbone = require('backbone');
-
-module.exports = Backbone.Model.extend({
+module.exports = require('backbone').Model.extend({
 
   idAttribute: 'src',
 

src/asset_manager/model/AssetImage.js

@@ -1,5 +1,4 @@
-var Backbone = require('backbone');
-var Asset = require('./Asset');
+const Asset = require('./Asset');
 
 module.exports = Asset.extend({
 

src/asset_manager/view/AssetView.js

@@ -1,5 +1,3 @@
-var Backbone = require('backbone');
-
 module.exports = Backbone.View.extend({
 
   initialize(o = {}) {

src/asset_manager/view/AssetsView.js

@@ -1,4 +1,3 @@
-var Backbone = require('backbone');
 var AssetView = require('./AssetView');
 var AssetImageView = require('./AssetImageView');
 var FileUploader = require('./FileUploader');