dhcp: T3316: Adjust dhcp-run script to align with kea hooks

The hook arguments passed to on-dhcp-event.sh have changed in Kea.
Adjust the script to align with the new arguments.

Additionally, remove FQDN mangling from the script. No need to extract
the domain name from LEASE4_HOSTNAME only to append it again.

See: https://kea.readthedocs.io/en/latest/arm/hooks.html#hooks-run-script