io_uring: remove unnecessary barrier after unsetting IORING_SQ_NEED_WAKEUP

There is no operation to order with afterwards, and removing the flag is
not critical in any way.

There will always be a "race condition" where the application will
trigger IORING_ENTER_SQ_WAKEUP when it isn't actually needed.

Signed-off-by: Stefan Bühler <source@stbuehler.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>