[May 18] Remove deprecated images from Docker Hub

See https://mail.gnome.org/archives/buildstream-list/2019-February/msg00047.html for details on deprecation and removals.

Based on the above, here are the tasks with relevant dates:

• 18th March 2019:

  • Remove buildstream/buildstream-fedora from CI pipelines - !114 (merged)
  • REMOVE buildstream/testsuite-debian from Docker Hub
  • REMOVE buildstream/testsuite-fedora from Docker Hub
  • REMOVE buildstream/testsuite-centos from Docker Hub
  • REMOVE buildstream/testsuite-ubuntu from Docker Hub
  • REMOVE buildstream/testsuite-ubuntu-18 from Docker Hub
  • REMOVE buildstream/testsuite-python from Docker Hub
  • REMOVE buildstream/buildstream-debian from Docker Hub

• 18th May 2019:

  • REMOVE buildstream/buildstream-fedora from Docker Hub