xfrm: policy: xfrm_policy_unregister_afinfo can return void

Nothing checks the return value. Also, the errors returned on unregister
are impossible (we only support INET and INET6, so no way
xfrm_policy_afinfo[afinfo->family] can be anything other than 'afinfo'
itself).

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>