io_uring/uring_cmd: remove dead req_has_async_data() check

Any uring_cmd always has async data allocated now, there's no reason to
check and clear a cached copy of the SQE.

Fixes: d10f19df ("io_uring/uring_cmd: switch to always allocating async data")
Signed-off-by: Jens Axboe <axboe@kernel.dk>