Amazon Pay ECE: Hide settings for legacy checkout

[annemirasol]

Changes proposed in this Pull Request:

Amazon Pay is not available for legacy checkout. This PR adds an extra check so we don't display it in the settings page if legacy checkout is enabled.

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. Enable legacy checkout.
3. Verify that Amazon Pay is not available under Payment Methods > Express Checkout.
4. Disable legacy checkout.
5. Verify that Amazon Pay is available under Payment Methods > Express Checkout.

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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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• Added testing instructions to the Release Testing Instructions wiki page (or does not apply)
• Added the needs docs label (or does not apply)
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[malithsen]

Thanks for the PR! Looks good and tests well