netfilter: cleanup unused macro

TEMPLATE_NULLS_VAL is not used after commit 0838aa7f
("netfilter: fix netns dependencies with conntrack templates")

PFX is not used after commit 8bee4bad ("netfilter: xt
extensions: use pr_<level>")

Signed-off-by: Li RongQing <lirongqing@baidu.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>