netfilter: nf_log_common: merge with nf_log_syslog

Remove nf_log_common.  Now that all per-af modules have been merged
there is no longer a need to provide a helper module.

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