Remove the custom PaymentType plugin and payment workflow

This module currently defines a custom PaymentType plugin that also uses a custom payment workflow. Absent a true need for custom fields and workflow steps, it's considered best practice to use the payment_default type and workflow.

When this workflow is removed, we should review the existing code to ensure the "failed" step it added is not expected to be used anywhere. (I don't think it should be; if at all, it would likely be addressed by changing when / how we create payments in #3366835: Do not create a Payment entity until a payment has succeeded.)