fix double delete in rebuild_refcounts
fix double delete in rebuild_refcounts

in case of the Manifest having an IntegrityError,
the entry for the manifest was already deleted.

RonnyPfannschmidt/borg,edgewood/borg,edgewood/borg,RonnyPfannschmidt/borg,RonnyPfannschmidt/borg,RonnyPfannschmidt/borg,RonnyPfannschmidt/borg,edgewood/borg,edgewood/borg