Remove unused sort criteria from the Presentations grid

Presentations are currently being displayed in the same order than they appear in the Presentations folder but the catalog search has a sort key set.

This sort key has no effect because it is trying to use an index that does not exist. But it has to be removed because as soon as there is an index with the name specified in this catalog search, Plone will try to use it and if it does not work (the index does not have any entry) the search will not return results