io_uring: remove req cancel in ->flush()

Every close(io_uring) causes cancellation of all inflight requests
carrying ->files. That's not nice but was neccessary up until recently.
Now task->files removal is handled in the core code, so that part of
flush can be removed.

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