Fix GTK assert triggered in wxNotebook::DoRemovePage(). GTK unparents the
page, the caller should not do it.

This is a backport form 2.8.0.
--- src/gtk/notebook.cpp.orig	2007-03-14 09:43:15.000000000 -0400
+++ src/gtk/notebook.cpp	2007-03-14 09:41:58.000000000 -0400
@@ -599,7 +599,9 @@
 
     gtk_widget_ref( client->m_widget );
     gtk_widget_unrealize( client->m_widget );
-    gtk_widget_unparent( client->m_widget );
+
+    // we don't need to unparent the client->m_widget; GTK+ will do
+    // that for us (and will throw a warning if we do it!)
 
     // gtk_notebook_remove_page() sends "switch_page" signal with some strange
     // new page index (when deleting selected page 0, new page is 1 although,