Revert "network/ndisc: ignore most fields of RA header when lifetime is zero"

Revert "network/ndisc: ignore most fields of RA header when lifetime is zero"

This reverts commit ffef01acddcac27caaef02f7f28bff03d7799e5e.

Similar to 2d393b1b6d8 ("network: IPv6 Compliance: Router Advertisement
Processing, Reachable Time [v6LC.2.2.15]"),

Extract from: https://www.ietf.org/rfc/rfc4861.html#section-4.2, p.21,
first paragraph:

    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.

So it does not make sense to prevent DHCPv6 when Router Lifetime is 0.

Fixes #33357.