gtk: rely on default marshallers

Similar to previous removals of g_cclosure_marshal_VOID__VOID we can remove
other marshallers for which are a simple G_TYPE_NONE with single parameter.
In those cases, GLib will setup both a c_marshaller and va_marshaller for
us. Before this commit, we would not get a va_marshaller because the
c_marshaller is set.

Related to GNOME/Initiatives#10

gtk/gtknativedialog.c

@@ -280,7 +280,7 @@ gtk_native_dialog_class_init (GtkNativeDialogClass *class)
                   G_SIGNAL_RUN_LAST,
                   G_STRUCT_OFFSET (GtkNativeDialogClass, response),
                   NULL, NULL,
-                  g_cclosure_marshal_VOID__INT,
+                  NULL,
                   G_TYPE_NONE, 1,
                   G_TYPE_INT);
 }