io_uring: remove empty cleanup of OP_OPEN* reqs

A switch in __io_clean_op() doesn't have default, it's pointless to list
opcodes that doesn't do any cleanup. Remove IORING_OP_OPEN* from there.

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