io_uring: remove unused park_task_work

As sqpoll cancel via task_work is killed, remove everything related to
park_task_work as it's not used anymore.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/310d8b76a2fbbf3e139373500e04ad9af7ee3dbb.1621201931.git.asml.silence@gmail.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>