Fix unwanted focus events when deleting a wxTreeCtrl item in wxMSW.

Disable setting focus in response to TVN_SELCHANGING when deleting an item in
a single selection control too -- this was already done in multi selection
case but not in this one, for some reason.

See #15721.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75306 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775