netfilter: cttimeout: remove set but not used variable 'l3num'

Fixes gcc '-Wunused-but-set-variable' warning:

net/netfilter/nfnetlink_cttimeout.c: In function 'cttimeout_default_set':
net/netfilter/nfnetlink_cttimeout.c:353:8: warning:
 variable 'l3num' set but not used [-Wunused-but-set-variable]

It not used any more after
commit dd2934a9 ("netfilter: conntrack: remove l3->l4 mapping information")

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>