io_uring: drop any code related to SCM_RIGHTS

Commit 6e5e6d274956305f1fc0340522b38f5f5be74bdb upstream.

This is dead code after we dropped support for passing io_uring fds
over SCM_RIGHTS, get rid of it.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <sashal@kernel.org>