This is the final step in reducing disk utilization on this smaller
backup server by removing unneeded backups. We purge (delete) the
backups for the recently retired (keep a single backup) backups. This
should free up even more space giving us further headroom between
prunes.
Change-Id: I8324acbe69a09d400f1bdbd26c806b5cb5233719
ff8cc2eec0