client: Remove WL_ZOMBIE_OBJECT global

Since we now have the WL_MAP_ENTRY_ZOMBIE flag to determine whether or
not a client-side object is a zombie, we can remove the faux object.

[daniels: Extracted from Derek's bespoke-zombie patch as an intermediate
          step.]

Signed-off-by: Derek Foreman <derekf@osg.samsung.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>