client: remove two unused function pointer typedefs

The need for wl_display_update_func_t was removed in

commit 53d24713
Author: Kristian Høgsberg <krh@bitplanet.net>
Date:   Thu Oct 4 16:54:22 2012 -0400

    Change filedescriptor API to be thread safe

and wl_callback_func_t does not seem to have ever been used in the first place.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>