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: NColin Ian King <colin.king@canonical.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -4956,7 +4956,7 @@ static int io_poll_double_wake(struct wait_queue_entry *wait, unsigned mode,
 
 	list_del_init(&wait->entry);
 
-	if (poll && poll->head) {
+	if (poll->head) {
 		bool done;
 
 		spin_lock(&poll->head->lock);