Simplify exchange variant callback and prefer delete over destroy

Delete is faster than destroy but should only be used if the object has no callbacks or touches.

app/models/exchange_variant.rb

@@ -3,9 +3,13 @@
 class ExchangeVariant < ApplicationRecord
   belongs_to :exchange
   belongs_to :variant, class_name: 'Spree::Variant'
-  after_destroy :destroy_related_outgoing_variants
 
-  def destroy_related_outgoing_variants
-    VariantDeleter.new.destroy_related_outgoing_variants(variant_id, exchange.order_cycle)
+  after_destroy :delete_related_outgoing_variants
+
+  def delete_related_outgoing_variants
+    ExchangeVariant.where(variant_id: variant_id).
+      joins(:exchange).
+      where(exchanges: { order_cycle: exchange.order_cycle, incoming: false }).
+      delete_all
   end
 end

app/services/variant_deleter.rb

@@ -11,15 +11,6 @@ class VariantDeleter
     variant.destroy
   end
 
-  def destroy_related_outgoing_variants(variant_id, order_cycle)
-    internal_variants = ExchangeVariant.where(variant_id: variant_id).
-      joins(:exchange).
-      where(
-        exchanges: { order_cycle: order_cycle, incoming: false }
-      )
-    internal_variants.destroy_all
-  end
-
   private
 
   def only_variant_on_product?(variant)