Remove nonsensical custom comparison code from wxDataViewIndexListModel.

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