xfrm: policy: remove last remnants of pernet inexact list

xfrm_net still contained the no-longer-used inexact policy list heads,
remove them.

Fixes: a54ad727 ("xfrm: policy: remove remaining use of inexact list")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>