net: remove cmsg restriction from io_uring based send/recvmsg calls

No need to restrict these anymore, as the worker threads are direct
clones of the original task. Hence we know for a fact that we can
support anything that the regular task can.

Since the only user of proto_ops->flags was to flag PROTO_CMSG_DATA_ONLY,
kill the member and the flag definition too.

Signed-off-by: Jens Axboe <axboe@kernel.dk>