io_uring: remove dead non-zero 'poll' check

Colin reports that Coverity complains about checking for poll being
non-zero after having dereferenced it multiple times. This is a valid
complaint, and actually a leftover from back when this code was based
on the aio poll code.

Kill the redundant check.

Link: https://lore.kernel.org/io-uring/fe70c532-e2a7-3722-58a1-0fa4e5c5ff2c@canonical.com/


Reported-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>