[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