Remove wxHeaderColumn::SetAsSortKey() and only use SetSortOrder().

Remove wxHeaderColumn::SetAsSortKey() and only use SetSortOrder().

The two member functions, SetAsSortKey() and SetSortOrder(), were doing almost
the same thing but differently and the former was only used in the generic
wxDataViewCtrl implementation and not implemented in the native GTK/OS X one.

Remove SetAsSortKey() entirely and only keep UnsetAsSortKey() which is still
needed by generic/MSW wxDataViewCtrl. But only SetSortOrder() should now be
called to indicate that the column is used for sorting.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69174 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775