io_uring: remove extra io_wq_current_is_worker()

io_wq workers use io_issue_sqe() to forward sqes and never
io_queue_sqe(). Remove extra check for io_wq_current_is_worker()

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>