rm'ing the holding dir should work fine.  run amadmin config find
before and after and you'll note that the disks are recorded as being
in the holding dir, but not assigned to a tape.

The only issue I can see is getting amanda's dump levels and
/etc/dumpdates out of sync, but if these are old and you have more
recent level 0s, that won't matter.  I would guess that's the case or
you wouldn't want to delete them...

-- 
        Greg Troxel <gdt AT ir.bbn DOT com>