Remove nonsensical custom comparison code from wxDataViewIndexListModel.

This model was comparing its items by their positions in the list instead of
their values for some reason. This broke sorting in wxDataViewListCtrl using
this model and simply didn't make any sense.

Just remove the comparison code from wxDataViewIndexListModel entirely, the
base class version works just fine for it.

Closes #14116.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71055 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/dataview.h

@@ -376,12 +376,6 @@ public:
     virtual unsigned GetRow( const wxDataViewItem &item ) const;
     wxDataViewItem GetItem( unsigned int row ) const;
 
-    // compare based on index
-
-    virtual int Compare( const wxDataViewItem &item1, const wxDataViewItem &item2,
-                         unsigned int column, bool ascending ) const;
-    virtual bool HasDefaultCompare() const;
-
     // implement base methods
     virtual unsigned int GetChildren( const wxDataViewItem &item, wxDataViewItemArray &children ) const;