listbase: Don't specify the same thing twice

We only need to set EXPLICIT_NOTIFY once.

Pointed out in https://www.viva64.com/en/b/0793/

gtk/gtklistbase.c

@@ -1148,7 +1148,7 @@ gtk_list_base_class_init (GtkListBaseClass *klass)
                        P_("The orientation of the orientable"),
                        GTK_TYPE_ORIENTATION,
                        GTK_ORIENTATION_VERTICAL,
-                       G_PARAM_READWRITE | G_PARAM_EXPLICIT_NOTIFY | G_PARAM_EXPLICIT_NOTIFY);
+                       G_PARAM_READWRITE | G_PARAM_EXPLICIT_NOTIFY);
 
   g_object_class_install_properties (gobject_class, N_PROPS, properties);