io_uring: remove dead 'poll_only' argument to io_poll_cancel()

It's only called from one location, and it always passes in 'false'.
Kill the argument, and just pass in 'false' to io_poll_find().

Signed-off-by: Jens Axboe <axboe@kernel.dk>
Link: https://lore.kernel.org/r/20220418164402.75259-2-axboe@kernel.dk