Using the discount code set on the level object when set

[dparker1005]

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Changes proposed in this Pull Request:

During asynchronous checkouts, we are currently saving the discount code used at checkout in an order meta field and also on the checkout level object. We have seen issues where those discount codes do not match which may cause discount code uses not to be saved.

To streamline our data storage and avoid duplicate (and potentially conflicting) information, this PR makes the discount code set on the checkout level the source of truth. In a future breaking update, discount code data will only be pulled from the checkout level object when completing checkout.

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Changelog entry

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