[FIX] sale, website_sale: move #buy_now disable

Some code was not correctly merged during forward port.

commit introducing the new method
https://github.com/odoo/odoo/commit/51ef8514eaddaebbdfee73230cfe955735d02f67

commit introducing the issue:
https://github.com/odoo/odoo/commit/71a50a2214748fec413c5a0f042d0e7172ae6ea9

addons/sale/static/src/js/variant_mixin.js

@@ -403,7 +403,6 @@ var VariantMixin = {
                 );
             });
         });
-        $parent.find("#buy_now").toggleClass('disabled', disable);
     },
     /**
      * Extracted to a method to be extendable by other modules

addons/website_sale/static/src/js/website_sale.js

@@ -698,6 +698,7 @@ publicWidget.registry.WebsiteSale = publicWidget.Widget.extend(VariantMixin, {
     _toggleDisable: function ($parent, isCombinationPossible) {
         VariantMixin._toggleDisable.apply(this, arguments);
         $parent.find("#add_to_cart").toggleClass('disabled', !isCombinationPossible);
+        $parent.find("#buy_now").toggleClass('disabled', !isCombinationPossible);
     },
     /**
      * Write the properties of the form elements in the DOM to prevent the