workqueue: remove unused WORK_CPU_END

WORK_CPU_END is totally unused since 4e8b22bd

 ("workqueue: fix
pool ID allocation leakage and remove BUILD_BUG_ON() in
init_workqueues"). It should be removed.

After it is removed, the comment "special cpu IDs" is not precise due to
there is only one special CPU ID (WORK_CPU_UNBOUND) left, so we also
change this comment to the description for WORK_CPU_UNBOUND.

tj: Minor description and comment tweaks.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>