Typo at Mapper.delete_obj

Using 2693 some delete operations fail with a {{{sqlalchemy.exceptions.ConcurrentModificationError: Updated rowcount 3 does not match number of objects updated 3 }}} triggered at {{{Module sqlalchemy.orm.mapper:1296 in delete_obj}}}.

Probably due a typo since the check which triggers the exception looks at {{{c.rowcount }} while the message looks at {{{c.cursor.rowcount}}} (hence the exception always says "x doesn't match x")

The attached path fixes it.