Remove CI jobs for building Ubuntu 16.04 packages

Questions we need to answer

1. Will GitLab.com fleet be upgraded from 16.04 by GitLab 13.11?
2. If not, how should we proceed?
   1. Should we provide packages only for GitLab.com and remove support for customers? - !5006 (comment 508097956)
   2. If not, do we provide both CE and EE packages, or just EE ones? - !5006 (comment 508120450)
3. what can we do/help to prioritize GitLab.com fleet upgrade and complete this?