Remove support for the deprecated reservation-mode config flag

As mentioned in the ARM:

Since Kea 1.9.1, reservation mode has been replaced by three boolean flags, reservations-global, reservations-in-subnet, and reservations-out-of-pool, which allow the configuration of host reservations both globally and in a subnet.

The deprecation happened in Kea 1.9. Kea 2.0 shipped with the deprecation warning in the ARM. Kea 2.1 could have had support for these parameters removed.