netfilter: nfnetlink_log: remove empty nfnetlink_log.h header file

/include/net/netfilter/nfnetlink_log.h file is empty.
so that it can be removed.

Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>