workqueue: Remove deprecated flush[_delayed]_work_sync()

flush[_delayed]_work_sync() were deprecated by 43829731

 ("workqueue:
deprecate flush[_delayed]_work_sync()") and have been deprecated
for a long time. In addition, these are not used anymore. So,
let's remove these functions.

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Tejun Heo <tj@kernel.org>