[FIX] website_sale: delete explicitely change return_path

In the case of deletion with `ondelete="set null"`, return_path of
children of deleted record will not have return_path updated.

So remove the parent explicitely so the ORM know to update them.

note: in master the parent_id field of product.public.category will be
set to `ondelete="cascade"`.

opw-2148046
closes odoo#41075

Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>

addons/website_sale/models/product.py

@@ -163,6 +163,10 @@ def name_get(self):
             res.append((category.id, " / ".join(category.parents_and_self.mapped('name'))))
         return res
 
+    def unlink(self):
+        self.child_id.parent_id = None
+        return super(ProductPublicCategory, self).unlink()
+
     def _compute_parents_and_self(self):
         for category in self:
             if category.parent_path: