gdk: Remove gdk_cairo_get_clip_rectangle()

gdk: Remove gdk_cairo_get_clip_rectangle()

The function is fundamentally broken for unbounded surfaces.
If a surface is unbounded, we cannot represent this as a
cairo_rectangle_int_t, and using the return value doesn't work because
it's already used for something else.

In GTK3, unbounded surfaces aren't a problem, but GTK4 uses recording
surfaces.

So better remove that function before we keep using it and using it
wrong.