Don't rely on getting PAGE_CHANGED event in wxOSX wxNotebook

The user code may handle this event, meaning that its handler in wxNotebook
itself is never invoked, but this broke actually changing the pages.

Fix this by doing the page change in the code handling the underlying OS X
event directly.

Closes #17197.

include/wx/osx/notebook.h

@@ -99,7 +99,6 @@ public:
   // callbacks
   // ---------
   void OnSize(wxSizeEvent& event);
-  void OnSelChange(wxBookCtrlEvent& event);
   void OnSetFocus(wxFocusEvent& event);
   void OnNavigationKey(wxNavigationKeyEvent& event);