Fixes STRIPE-799

Changes proposed in this Pull Request:

Discussed here: p1762519898532299-slack-C055WHLA98D
There are some concerns that the redirect_url parameter provided to the ECE payment flow might not be safely stripped.
After testing and consulting the Stripe docs, it looks like it might not be needed.

Testing instructions

• Allow your site to use Amazon Pay with wp option update _wcstripe_feature_amazon_pay yes
• Navigate to WooCommerce > Settings > Payments > Stripe
• Scroll down to the "Express checkouts" section
• Enable Amazon Pay
• Save
• Click "Customize" on Amazon Pay
• Ensure it's enabled on Checkout, Cart, & Product Page locations
• As a customer, add a product to the cart (or attempt the checkout from the product page)
• Navigate to the Cart or Checkout pages
• Click on the Amazon Pay button
• If you don't have an account, create one (you can't login with your "real" Amazon account)
• Fill in the details, such as address & payment information
• Pay
• You should be redirected to the "Order received" page

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

Changelog entry

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Post merge

• Added testing instructions to the Release Testing Instructions wiki page (or does not apply)
• Added the needs docs label (or does not apply)
• Included this PR in the Release Thread scope (or does not apply)