Cleaning object require to load it, so if we can't load it during download,
we must postpone cleanup during installation, which has 2 drawbacks:
- One is temporary: the page showing changes during installation may list
  objects which have no useful change (because there is no more clean up before
  comparing objects)
- During installation, Python Scripts are compiled twice instead of one (this
  commit partially reverts r38348), but it seems fixable.

This fix regressions caused by r38601.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@38632 20353a03-c40f-0410-a6d1-a30d3c3de9de