Ancestors table hangs on to foreign key relationships if it can't be deleted

If the upgrade task fails to remove the ancestors table for some reason, then it will hang on to foreign key relationships with the content table, making it impossible to delete content.

We should issue a "delete from ANCESTORS" before we try to drop it, just in case the drop fails. Although why you'd be able to do one and not the other I'm not sure of, but there's been one support case in which the table wasn't dropped, and it doesn't hurt to be sure.