Reset sorting column index in generic wxDataViewCtrl when clearing columns.

Otherwise we could continue to use the now invalid index for sorting.

Closes #15803.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75533 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775