Amazon Pay ECE: update selected payment method

[annemirasol]

Part of #3859
Builds on changes introduced in #4015 and #4022
Prerequisite for #4008

Changes proposed in this Pull Request:

As part of supporting subscriptions and saved payment methods for Amazon Pay, we want Amazon Pay orders to use amazon_pay instead of card as the selected payment method, for the following reasons:

• When crafting payment method options, we need to use amazon_pay instead of card. Otherwise, the payment method will not attach to the customer.
• We need to have a separate logic when creating payment tokens. Amazon Pay does not expose card details such as last 4 digits or brand, so we cannot use the existing payment token logic for cards.

This PR also removes overrides and custom logic that are no longer necessary:

• Payment method title overrides. No longer necessary as Amazon Pay is now its own payment method and no longer riding on the card payment method.
• Code for warning shoppers regarding delayed payment verification. No longer necessary as Amazon Pay does not have to be async after all.

Testing instructions

1. Turn on the Amazon Pay feature flag using the Stripe helper plugin, or by setting _wcstripe_feature_amazon_pay to yes in wp_options.
2. As a shopper, purchase any product using Amazon Pay.
3. In the Order Received confirmation page, verify that the payment method correctly says "Amazon Pay".
4. In wp-admin Edit Order page, verify that the order says "Payment via Amazon Pay".
5. In Stripe settings, enable manual capture, i.e. Issue an authorization on checkout, and capture later.
6. As a shopper, make another purchase via Amazon Pay.
7. As the merchant, manually capture the charge by setting the order to "Processing".
8. Verify that the charge was successfully captured.
   • Under Order Notes, you will see a Stripe charge complete (Charge ID: py_xxxx) note.

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• Covered with tests (or have a good reason not to test in description ☝️)
• Tested on mobile (or does not apply)

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[malithsen]

👍 Code looks good and tests well. Thanks for the PR!