xfrm: policy: remove remaining use of inexact list

No consumers anymore, remove it.  After this, insertion of policies
no longer require list walk of all inexact policies but only those
that are reachable via the candidate sets.

This gives almost linear insertion speeds provided the inserted
policies are for non-overlapping networks.

Before:
Inserted 1000   policies in 70 ms
Inserted 10000  policies in 1155 ms
Inserted 100000 policies in 216848 ms

After:
Inserted 1000   policies in 56 ms
Inserted 10000  policies in 478 ms
Inserted 100000 policies in 4580 ms

Insertion of 1m entries takes about ~40s after this change
on my test vm.

Cc: Noel Kuntze <noel@familie-kuntze.de>
Cc: Tobias Brunner <tobias@strongswan.org>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>