Identify and delete a refund when an async refund failure occurs

[reykjalin]

Currently, when refunds fail asynchronously, the WC refund is still present in the Order Details page. We should try to delete the WC refund so it's not present on the order after a refund has failed.

I think can probably (identify and) delete the refund when an async refund failure comes in, as is done in the sync refund failure case in WC core [source] – we can open a separate issue for that though, since it's not strictly related to the change in this PR.

Originally posted by @dechov in #2167 (comment)

I looked into this briefly, and I'm pretty sure we'll need to add some meta data to either the main WC_Order or the WC_Order_Refund so we can identify the refund we should be deleting.

Reproduction steps

1. Pay for an order with the 4000 0000 0000 5126 testing card
2. Open that order's details page
3. Refund any non-zero amount for the order
4. The refund will fail asynchronously which means:
   • You have to wait 5-10 seconds and then reload the page to see the refund failure.
   • You have to make sure webhooks work
5. Now that you've reloaded the page notice that the refund failed (there should be an order note) and the WC refund still exists in the list of items associated with the WC Order.