xfrm: Remove caching of xfrm_policy_sk_bundles

We currently cache socket policy bundles at xfrm_policy_sk_bundles.
These cached bundles are never used. Instead we create and cache
a new one whenever xfrm_lookup() is called on a socket policy.

Most protocols cache the used routes to the socket, so let's
remove the unused caching of socket policy bundles in xfrm.

Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>