[REF] website_slides: remove index_content field from slide.slide

The 'index_content' field was only filled when uploading from the frontend.
That would lead to inconsistent results when creating/updating 'presentation' slides from the slide.slide form view.
The field was also not cleaned if the slide content changed.

As it's usage was not very clear (only on a 'search', and not very intuitive), we simply removed the field.