Build only 16.04 package for automatic deploys
We still build both 18.04 and 16.04 for any autodeploy package, eg https://dev.gitlab.org/gitlab/omnibus-gitlab/pipelines/133835
We do not have any 18.04 nodes for GitLab.com so creating this package only delays things unnecessarily and this was obviously added as a premature optimisation.
We should only build 16.04 packages.