relay: Simplify IPv6 discovery when building descriptor

Now that relay_find_addr_to_publish() checks if we actually have an ORPort, we
can simplify the descriptor building phase for IPv6.

This also avoid triggering an IPv6 discovery if the IPv4 can't be found in the
first place.

Related to #40254

Signed-off-by: David Goulet <dgoulet@torproject.org>