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>

include/linux/workqueue.h

@@ -56,9 +56,8 @@ enum {
 	WORK_NR_COLORS		= (1 << WORK_STRUCT_COLOR_BITS) - 1,
 	WORK_NO_COLOR		= WORK_NR_COLORS,
 
-	/* special cpu IDs */
+	/* not bound to any CPU, prefer the local CPU */
 	WORK_CPU_UNBOUND	= NR_CPUS,
-	WORK_CPU_END		= NR_CPUS + 1,
 
 	/*
 	 * Reserve 7 bits off of pwq pointer w/ debugobjects turned off.