inet6: Stop listening to NA messages

inet6: Stop listening to NA messages

This is very expensive, and we only listen for them for the
Router -> Host flag change. Linux and now NetBSD announce this
change via the netlink/routing socket so there is no longer a
need to listen for it.

RFC 4861 6.2.5 only says departing routers *SHOULD* send RA
with lifetime of zero and *MUST* send all subsequent NA messages
with the router flag unset, so dhcpcd *SHOULD* still work.

For the corner case where it doesn't, consider patching your
kernel.