auth.idl: make sure ndr_{push,pull}_auth_SidAttr() is never used

auth.idl: make sure ndr_{push,pull}_auth_SidAttr() is never used

auth_SidAttr is currently not used for any IPC traffic,
with this change demonstrates that and makes sure it
stays that way.

It means we are free to change auth_SidAttr without the
need to change any ipc protocol version.

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Jennifer Sutton <jennifersutton@catalyst.net.nz>