net: remove obsolete members from struct net

all have been moved to generic_net infra. On x86_64, this reduces
struct net size from 70 to 63 cache lines (4480 to 4032 byte).

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