io_uring: remove unused hash_wait

No users of io_uring_ctx::hash_wait left, kill it.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/e25cb83c233a5f75f15275596b49fbafbea606fa.1617287883.git.asml.silence@gmail.com


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