Fix Spree issue https://github.com/spree/spree/issues/4771
When a tax rate and/or promotion is deleted we should not destroy any adjustment that was created already. The adjustments are snapshot of an order at the time when those adjustments were applied. Deleting tax rate and promotion should not affect the adjustments that are applied already even for incomplete order. It's bad user experience if we remove those adjustments silently. We just nullify the source_id.