workqueue: remove deprecated WQ_NON_REENTRANT

Tejun Heo has made WQ_NON_REENTRANT useless in the dbf2576e
("workqueue: make all workqueues non-reentrant"). So remove its
usages and definition.

This patch doesn't introduce any behavior changes.

tj: minor description updates.

Signed-off-by: ZhangZhen <zhenzhang.zhang@huawei.com>
Sigend-off-by: Tejun Heo <tj@kernel.org>
Acked-by: James Chapman <jchapman@katalix.com>
Acked-by: Ulf Hansson <ulf.hansson@linaro.org>