Tidy up of the unpacked.cern.ch wishlist

Dear All,

We start to notice abandoned container images on the wishlist for the unpacked.cern.ch repository. We would like to occasionally tidy up that list and remove images that do not exist anymore. As a result, the removed images will also disappear from cvmfs. First removals will start as of next week (see merge request). If you notice any issues, please let us know.