io-wq: remove unused busy list from io_sqe

Commit e61df66c ("io-wq: ensure free/busy list browsing see all
items") added a list for io workers in addition to the free and busy
lists, not only making worker walk cleaner, but leaving the busy list
unused. Let's remove it.

Signed-off-by: Hillf Danton <hdanton@sina.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>