kea-dhcpv6 lease sanity-checking is flagging PD leases

Reported via support issue https://support.isc.org/Ticket/Display.html?id=14522.

Analysis showed that kea-dhcpv6 sanity checking is attempting to map PD leases to subnets by address which a: is not supported b: the admin guide says is not done for PD leases. Testing confirms this as does visual code inspection. The code should either not check PD leases or check them properly. In either case, there are no unit tests to verify PD leases are treated correctly.