io_uring: kill REQ_F_CANCEL_SEQ

We removed the reliance on the flag by the cancellation path and now
it's unused.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/e57afe566bbe4fefeb44daffb08900f2a4756577.1721819383.git.asml.silence@gmail.com


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