address some rack deletion issues (#757)

This is likely to fix future removals of racks, but does not address
files and tags that could be left over past removals (FK exists between
Rack/Object and RackSpace tables only in this context).

* commitDeleteRack(): remove unused function, anyway it is broken since
  RackHistory table was dismissed
* deleteRack(): handle removal of associated data (files, tags and
  non-object rackspace)