Override ClearColumns() in wxDataViewListCtrl.

Without this, the associated store's columns would be out of sync,
causing asserts next time a column is appended.

Fixes #14952.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75258 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775