Some cleanup unused code and missing use-strict

This clean the javascript, where we forgot some use-strict, and remove some dead code.

You will notice the presence of an `Object.seal(this)` [MDN](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/seal) to extend the functionality of `"use strict";` by raising if we try to set the value of a non-existing property on an object. IE `notebook.kernelselector= 1` will raise, as only `kernel_selector` exists.

`save_notebook_error` is not defined. You need to trigger the error event:

```javascript
this.events.trigger('notebook_save_failed.Notebook',
    new Error("Load failed, save is disabled")
);