netfilter: conntrack: Remove nf_ct_conntrack_flush_report

The only user of nf_ct_conntrack_flush_report() was ctnetlink_del_conntrack().
After adding support for flushing connections with a given mark, this function
is no longer called.

Signed-off-by: Kristian Evensen <kristian.evensen@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>