role:swh_worker_azure: Remove nonexistent source for stretch release
role:swh_worker_azure: Remove nonexistent source for stretch release

Related T761

SoftwareHeritage/puppet-swh-site,SoftwareHeritage/puppet-swh-site,SoftwareHeritage/puppet-swh-site,SoftwareHeritage/puppet-swh-site,SoftwareHeritage/puppet-swh-site