Fix SPACK-93, SPACK-94, GitHub #150

- `remove_prefix` was modified to remove from the DB, but the package
  may not have been added to the DB yet when `remove_prefix` is called
  from `cleanup`.

- Made `remove_prefix` a pure utility function (it just removes the prefix)

- Added `installed_db.remove()` call only after the `remove_prefix` in
  `uninstall`.