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.

Also refactor the code to avoid duplicating TreeView_DeleteItem() calls.

Closes #15721.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75308 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/msw/treectrl.h

@@ -304,6 +304,9 @@ private:
     // item visually spans the entire breadth of the window then
     bool MSWIsOnItem(unsigned flags) const;
 
+    // Delete the given item from the native control.
+    bool MSWDeleteItem(const wxTreeItemId& item);
+
 
     // the hash storing the items attributes (indexed by item ids)
     wxMapTreeAttr m_attrs;