netfilter: ecache: remove nf_exp_event_notifier structure

Reuse the conntrack event notofier struct, this allows to remove the
extra register/unregister functions and avoids a pointer in struct net.

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