Everything docker/staging/production related is setting the SWH_CONFIG_FILENAME in the environment [1-5]
The tests now do as well.
Thus, this drops the no longer used other conditional paths from the mixin.
[1] prod/staging: https://forge.softwareheritage.org/source/puppet-swh-site/browse/production/?grep=SWH_CONFIG_FILENAME
[2] docker: https://forge.softwareheritage.org/source/swh-environment/browse/master/?grep=SWH_CONFIG_FILENAME
As sample:
[3] loader/lister: https://forge.softwareheritage.org/source/puppet-swh-site/browse/production/site-modules/profile/templates/swh/deploy/worker/swh-worker@.service.erb$10
[4] scheduler: https://forge.softwareheritage.org/source/swh-environment/browse/master/docker/docker-compose.yml$109
[5] api/rpc servers: https://forge.softwareheritage.org/source/puppet-swh-site/browse/production/site-modules/profile/manifests/swh/deploy/rpc_server.pp$104
Related to T1532