VC6 compilation fixes for wxTreeListCtrl.

For some reason comparing wxTreeListItem with its associated Type doesn't work
when using VC6 ("error C2593: 'operator ==' is ambiguous") so help the compiler
to do it.

Also remove the unnecessary (and unsupported by VC6) return from void function.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69220 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775