client: remove definition of wl_global

Nothing on the client side uses it since
9fe75537 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>
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passed #6278
build-native

00:01:57