workqueue: Remove outdated comment about exceptional workers in unbind_workers()

Long time before, workers are not ALL bound after CPU_ONLINE, they can
still be running in other CPUs before self rebinding.

But the commit a9ab775b ("workqueue: directly restore CPU affinity
of workers from CPU_ONLINE") makes rebind_workers() bind them all.

So all workers are on the CPU before the CPU is down.

And the comment in unbind_workers() refers to the workers "which are
still executing works from before the last CPU down" is outdated.
Just removed it.

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