netfilter: nf_log_ipv6: merge with nf_log_syslog

This removes the nf_log_ipv6 module, the functionality is now
provided by nf_log_syslog.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>