workqueue: Remove deprecated system_nrt[_freezable]_wq

system_nrt[_freezable]_wq were deprecated by 3b07e9ca

 ("workqueue:
deprecate system_nrt[_freezable]_wq") 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>