io_uring: remove timeout/poll specific cancelations

For historical reasons these were special cased, as they were the only
ones that needed cancelation. But now we handle cancelations generally,
and hence there's no need to check for these in
io_ring_ctx_wait_and_kill() when io_uring_try_cancel_requests() handles
both these and the rest as well.

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