[doc] Update for devstack configuration
[doc] Update for devstack configuration

Now that VPNaaS is running under L3 agent extension.
There is no need extra steps.

Change-Id: I0f6a6dfe83f81da9d98b7ba1bd1b594eed4152e8
Related-bug: 1692131

openstack/neutron-vpnaas,openstack/neutron-vpnaas