Revert "network: ndisc - drop routes of lifetime 0"

Revert "network: ndisc - drop routes of lifetime 0"

This reverts commit 4ccd48cb2693a3603f14a30b6286228597285d92.

From RFC 4861 section 4.2.
> The Router Lifetime applies only to the router's usefulness as a default
> router; it does not apply to information contained in other message fields
> or options.  Options that need time limits for their information include
> their own lifetime fields.

Hence, based on the lifetime field of the message header, we should not
drop any other information.
Note that, in ndisc_router_process_default(), we have already dropp the
default gateway when the lifetime is zero. Hence, we can safely drop the
change.