xt_cluster supersedes ipt_CLUSTERIP since it can be also used in
gateway configurations (not only from the backend side).

ipt_CLUSTER is also known to leak the netdev that it uses on
device removal, which requires a rather large fix to workaround
the problem: http://patchwork.ozlabs.org/patch/358629/

So let's deprecate this so we can probably kill code this in the
future.
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>