[FIX] website_sale: delete child category on parent

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 children recursively.

Another fix was done in 13.0 to avoid most issue in 6d5cd8b1.

opw-2148046
closes #41075

closes odoo/odoo#41178

X-original-commit: 6d5cd8b1
Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>