Delete stock_items for master variants

All variants have stock_items records, but master variants never use them, so these were always redundant.

db/migrate/20230605133804_remove_master_variants.rb

@@ -7,6 +7,7 @@ class RemoveMasterVariants < ActiveRecord::Migration[7.0]
     handle_master_line_items
     delete_master_inventory_units
     delete_master_variant_prices
+    delete_master_stock_items
     delete_master_variants
   end
 
@@ -53,6 +54,16 @@ class RemoveMasterVariants < ActiveRecord::Migration[7.0]
     )
   end
 
+  def delete_master_stock_items
+    ActiveRecord::Base.connection.execute(<<-SQL
+      DELETE FROM spree_stock_items
+      USING spree_variants
+      WHERE spree_variants.is_master = true
+        AND spree_variants.id = spree_stock_items.variant_id
+    SQL
+    )
+  end
+
   def delete_master_variants
     ActiveRecord::Base.connection.execute(<<-SQL
       DELETE FROM spree_variants