Remove support for the deprecated dhcp-ddns config flags

As mentioned in the ARM:

[the D2 parameters] have been moved out of the dhcp-ddns section so that they may be specified at the global, shared-network, and/or subnet levels.

The deprecation happened in Kea 1.7. Kea 1.8 shipped with the deprecation warning in the ARM. Kea 1.9 could have had support for these parameters removed.

As a side note, the quotes statement contradicts another one in the ARM that says:

These parameters can only be specified within the top-level dhcp-ddns section in the kea-dhcp4 configuration.

Also ncr-format" has an extraneous quote around that part of the ARM.