Don't use association in batch_builder

Previously this was building codes on the association and then relying
on the autosave behaviour to save the new records when save was called
on the Promotion.

It's simpler to just explicitly create them.

core/app/models/spree/promotion_code/batch_builder.rb

@@ -37,16 +37,13 @@ def generate_random_codes
         codes_for_current_batch += get_unique_codes(new_codes)
       end
 
-      codes_for_current_batch.map do |value|
-        promotion.codes.build(value: value, promotion_code_batch: promotion_code_batch)
+      codes_for_current_batch.each do |value|
+        Spree::PromotionCode.create!(
+          value: value,
+          promotion: promotion,
+          promotion_code_batch: promotion_code_batch
+        )
       end
-
-      promotion.save!
-
-      # We have to reload the promotion because otherwise all promotion codes
-      # we are creating will remain in memory. Doing a reload will remove all
-      # codes from memory.
-      promotion.reload
     end
   end