Revert "workqueue: restructure pool / pool_workqueue iterations in freeze/thaw functions"

This reverts commit 4146e8df5cce7b1800eb272387d5c1889719626e.
Revert "workqueue: introduce for_each_pool()"

This reverts commit 4d872295790f98e4d4ea5a06cda366a40475bdbd.