Clean up Scald DnD library to use exposed sort

The library including in version up to 7.x-1.0-beta4 is using a workaround based on click sortable field to emulate exposed sort. That was introduced in the 6.x branch to work with Views 2, which didn't support exposing a sort criteria.

Views 3 handle that just fine, which means that the workaround can be removed, and the default view changed to include available sorts in the Sort section, instead of relying on the Fields one and limiting the list to click sortable fields.