netfilter: conntrack: remove packet hotpath stats

These counters sit in hot path and do show up in perf, this is especially
true for 'found' and 'searched' which get incremented for every packet
processed.

Information like

searched=212030105
new=623431
found=333613
delete=623327

does not seem too helpful nowadays:

- on busy systems found and searched will overflow every few hours
(these are 32bit integers), other more busy ones every few days.

- for debugging there are better methods, such as iptables' trace target,
the conntrack log sysctls.  Nowadays we also have perf tool.

This removes packet path stat counters except those that
are expected to be 0 (or close to 0) on a normal system, e.g.
'insert_failed' (race happened) or 'invalid' (proto tracker rejects).

The insert stat is retained for the ctnetlink case.
The found stat is retained for the tuple-is-taken check when NAT has to
determine if it needs to pick a different source address.

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