DemoRequest: empty demo file & clean up

- Removed unused jquery version (was there any reason why we need that?)

- Is it possible to add an empty instance.js file to index.php? Makes it much easier to maintain demo and doesn't hurt anyone. If this is merged, I can hook in very easily without any modification on the core, like:

```javascript
codiad.demo = {
        init: function() {
            $('input[name=username]').val('demo');
            $('input[name=password]').val('demo');
            $('#context-menu a[onclick="codiad.filemanager.uploadToNode($(\'#context-menu\').attr(\'data-path\'));"]').attr("onclick", "codiad.demo.denied();");
        },
        
        denied: function() {
            codiad.message.error.('Not Allowed in Demo Mode');
            return false;
        }
```

This would be also very good for other hosters which want to restrict something