[migration] Drop cw_schema relation first without this, we ends up with the traceback shown at https://www.cubicweb.org/ticket/16130960. This is not the proper fix, which I have not been able to find. It seems due to this very rare case of deletion of such relation linked to CWRType vs order of execution of operation (in this case, the operation deleting the entity table is run before some other queries using it). As forcing this relation to be deleted before the entity type fixes the problem while this case seems rare enough, IMO this patch is "good enough". Closes #16130960