client: remove definition of wl_global

Nothing on the client side uses it since
9fe75537ad207c1496e6d9be41a8f5af4b876506 which was just before the 0.99
release.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-By: Markus Ongyerth <wl@ongy.net>