Data inview: Deleted panel example

Deleted the panel example for the following reasons:
* Custom overlay that is not supported by either overlay.js or Magnific Popup
* Does not have the appearance of a traditional panel (looks more like a popup)
* Lacked any controls
* Did not make sense from a usability perspective (since just blocks the viewport)
* Will be replaced by a more appropriate example at a later date (possibly involving a button)

Left the related JS in since it is generic enough to be used for other use cases.