xfrm: remove flow cache

After rcu conversions performance degradation in forward tests isn't that
noticeable anymore.

See next patch for some numbers.

A followup patcg could then also remove genid from the policies
as we do not cache bundles anymore.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>