async: remove unused @node from struct async_domain

The @node in struct async_domain is unused after we introduce
async_global_pending, remove it.

tj: Unnecessary whitespace adjustments dropped.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Arjan van de Ven <arjan@linux.intel.com>