Remove `datePaid` dependence from `isEligibleForRefund`

[iamgabrielma]

At the moment, to know if an Order is eligible for a refund, we perform a datePaid nil-check as one of the variables to return it.

We should explore using other options to avoid problems when a merchant changes order status manually, or when plugins alter this functionality, like the Order's needsPayment property instead.