Remove touch

I think I was just following convention from existing relationships. Perhaps you could argue that a variant is affected by the links added/removed.. but we never look at updated_at so really there's no point at all.

app/models/variant_link.rb

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
 class VariantLink < ApplicationRecord
-  belongs_to :source_variant, class_name: 'Spree::Variant', touch: true
-  belongs_to :target_variant, class_name: 'Spree::Variant', touch: true
+  belongs_to :source_variant, class_name: 'Spree::Variant'
+  belongs_to :target_variant, class_name: 'Spree::Variant'
 end