BusinessTemplate: clean up imported objects during installation instead of download

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