Remove notification_driver option

The Ceilometer integration for Nova uses notification_driver
parameter at the moment.
According to the logs this is deprecated:
Option "notification_driver" from group "DEFAULT" is deprecated.
Use option "driver" from group "oslo_messaging_notifications".

Change-Id: Ifb60a7a5945838bb8d12092d811956f84775df27
Closes-Bug: 1602667

ansible/roles/nova/templates/nova.conf.j2

@@ -55,7 +55,6 @@ my_ip = {{ hostvars[inventory_hostname]['ansible_' + api_interface]['ipv4']['add
 instance_usage_audit = True
 instance_usage_audit_period = hour
 notify_on_state_change = vm_and_task_state
-notification_driver = messagingv2
 {% endif %}
 
 {% if nova_console == 'novnc' %}