Reset sorting column in generic wxDataViewCtrl properly.

We could keep using the column previously used for sorting even after
UnsetAsSortKey() was called on it. Ensure that this doesn't happen by
resetting the owner wxDataViewCtrl sort column index too.

Closes #15160.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73834 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775