web: use setEmptyCriterionValid accessor directly

The accessor was added in 99cd5823 (erp5_property_sheets: Add
Predicate/empty_criterion_valid_property ., 2013-11-20) it's been
long enough to assume that every site have this property now.

bt5/erp5_web/SkinTemplateItem/portal_skins/erp5_web/WebSection_init.py

 # List no document until at least one Criterion is created.
-# BBB: property does not exist on old property sheets
-getattr(context, 'setEmptyCriterionValid', lambda _: None)(False)
+context.setEmptyCriterionValid(False)