`devture_traefik_provider_configuration_extension_yaml` should not be
set automatically by the playbook. It's a variable to be used by users.

Moreover, setting for for everyone (not just people who have additional JVBs)
means that for most people the following error will be produced:

> 'dict object' has no attribute 'jitsi_jvb_servers'

.. as detailed in https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/2844

Fixes a regression introduced in: https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/2842