Ensure jscroll instance is destroyed if the associated div is removed from the DOM.

If this is not done, and the div is updated/replaced in any way other than a
full browser page reload then the jscroll instance will be invalidated
and will prevent a new one being created. From an end-user perspective
JScroll would simply appear to have stopped working and wouldn't
resume working until the page was forcibly reloaded.

This made JScroll incompatible with various AJAX based methods/jQuery
plugins that would update the page content without reloading the entire page.