client: Use refcount exclusively for destruction

Commit e273c7cd added a refcount to wl_proxy. The refcount is set to 1
on creation, decreased when the client explicitly destroys the proxy,
and is increased and decreased every time an event referencing that
proxy is queued.

Assuming no bugs, this means the refcount cannot reach 0 without the
proxy being explicitly destroyed. However, some (not all) of the
proxy-unref paths were only destroying the proxy if it had already been
deleted. This should already be enforced by refcounting, so remove the
check and rely solely on the refcount as the arbiter of when to free a
proxy.

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