resolved-dns-query: remove dns_query_candidate_is_routable

resolved-dns-query: remove dns_query_candidate_is_routable

Resolved can't reliably determine on whether "it makes sense" to query
AAAA records when not explicitly specifying it in the request, so we
shouldn't remove them.

After having done the resolving, applications can use RFC6724 to
determine whether that address is reachable.

We can't know whether an address is reachable before having resolved it
and inspecting the routing table, and not resolving AAAA just because
there's no IPv6 default route on the main interface link them breaks
various setups, including IPv6-providing wireguard tunnels on a
non-dualstacked environment.

Fixes #5782
Fixes #5915
Fixes #8017