CLEANUP: tasks: remove the long-unused work_lists

CLEANUP: tasks: remove the long-unused work_lists

Work lists were a mechanism introduced in 1.8 to asynchronously delegate
some work to be performed on another thread via a dedicated task.
The only user was the listeners, to deal with the queue. Nowadays
the tasklets have made this much more convenient, and have replaced
work_lists in the listeners. It seems there will be no valid use case
of work lists anymore, so better get rid of them entirely and keep the
scheduler code cleaner.