netfilter: ip_tables: remove clusterip target

Marked as 'to be removed soon' since kernel 4.1 (2015).
Functionality was superseded by the 'cluster' match, added in kernel
2.6.30 (2009).

clusterip_tg_check still has races that can give

 proc_dir_entry 'ipt_CLUSTERIP/10.1.1.2' already registered

followed by a WARN splat.

Remove it instead of trying to fix this up again.
clusterip uapi header is left as-is for now.

Signed-off-by: Florian Westphal <fw@strlen.de>