There's no reason for DestroyTreeHelper() code, that's what destructors
are for. On a related note, make removal of children from a node more
explicit too.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68930 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775