io_uring: remove REQ_F_IO_DRAINED

A request can get into the defer list only once, there is no need for
marking it as drained, so remove it. This probably was left after
extracting __need_defer() for use in timeouts.

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