Remove obsolete notes about bulk lease queries from ARM

The ARM contains two notes, one for v4 and one for v6, that state "The new query types are only supported with the memfile lease backend". The query types referenced are also supported by MySQL and PostgreSQL back ends. These notes are obsolete and should be removed from the following sections:

https://kea.readthedocs.io/en/kea-2.6.0/arm/hooks.html#dhcpv4-bulk-leasequery

https://kea.readthedocs.io/en/kea-2.6.0/arm/hooks.html#dhcpv6-bulk-leasequery

We recently had a support customer change their configuration based on this note.