Remove inventory units related to master variants

These shouldn't technically exist, but apparently they can be present if the dataset is old enough. They can trigger a foreign key violation if they are present when a master variant is deleted, so they need to be dropped if present.

db/migrate/20230605133804_remove_master_variants.rb

@@ -4,12 +4,23 @@ class RemoveMasterVariants < ActiveRecord::Migration[7.0]
       delete_master_option_values
     end
 
+    delete_master_inventory_units
     delete_master_variant_prices
     delete_master_variants
   end
 
   private
 
+  def delete_master_inventory_units
+    ActiveRecord::Base.connection.execute(<<-SQL
+      DELETE FROM spree_inventory_units
+      USING spree_variants
+      WHERE spree_variants.is_master = true
+        AND spree_variants.id = spree_inventory_units.variant_id
+    SQL
+    )
+  end
+
   def delete_master_option_values
     ActiveRecord::Base.connection.execute(<<-SQL
       DELETE FROM spree_option_values_variants