Selection: Remove harmful assertion for existing sites

This assertion for newly created site probably ok, but this assertion for
the exisiting sites that is older than the following commit, we get serious
problem. We can not even look at the document through the UI because of the
assertion error.

commit 8df1f1439b8f8ecb64c906044633909ca5365046
Date:   Fri Apr 5 19:32:16 2013 +0200
ERP5Form: fix name of newly-created selections for listboxes that are not rendered in HTML