Update soft-delete of variants

Spree changed their way of soft-deleting variants from calling `delete`
to calling `destroy`. We don't need our own implementation any more.

app/models/spree/variant_decorator.rb

@@ -105,14 +105,6 @@ Spree::Variant.class_eval do
     OpenFoodNetwork::EnterpriseFeeCalculator.new(distributor, order_cycle).fees_by_type_for self
   end
 
-  def delete
-    transaction do
-      self.update_column(:deleted_at, Time.zone.now)
-      ExchangeVariant.where(variant_id: self).destroy_all
-      self
-    end
-  end
-
   def refresh_products_cache
     if is_master?
       product.refresh_products_cache

app/services/variant_deleter.rb

@@ -6,7 +6,7 @@ class VariantDeleter
       return false
     end
 
-    variant.delete
+    variant.destroy
   end
 
   private