io-wq: remove now redundant struct io_wq_nulls_list

Since we don't iterate these lists anymore after commit:

e61df66c ("io-wq: ensure free/busy list browsing see all items")

we don't need to retain the nulls value we use for them. That means it's
pretty pointless to wrap the hlist_nulls_head in a structure, so get rid
of it.

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