Remove Gdk.Rectangle alias with newer gobject-introspection and GTK+

The new GdkRectangle in the typelib confuses the marshalling code
as PyGObject uses the Python class from the overrides for marshalling
to Python but uses the gtype from the typelib to do
type checking when marshalling from Python.

https://bugzilla.gnome.org/show_bug.cgi?id=749625