Removing legacy payment method classes

[wjrosa]

Changes proposed in this Pull Request:

In this simple cleanup PR, I am removing all the legacy checkout payment method classes, as we are no longer using them.

Testing instructions

Code review. Check if the tests are still passing. Perform some basic smoke testing and confirm no regression was introduced.

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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]

Looks good. Are we planning to do a similar cleanup for the frontend code (to remove is_upe_enabled distinction) in a separate PR?

[daledupreez]

I have some initial comments from inspection, and I will move to test this later this morning. That said, it feels like it might be worth splitting this PR up a bit so we can tackle more specific and focused testing.

[daledupreez]

Might it be worth using a new constant for this rather than having 'stripe_sepa' everywhere?

It also feels like it could be worth pushing just that change into a dedicated PR, as that would be easy to inspect and review, and we could get it shipped with minimal risk. That would also reduce the size of this PR by quite a few files.

[wjrosa]

Looks good. Are we planning to do a similar cleanup for the frontend code (to remove is_upe_enabled distinction) in a separate PR?

@malithsen yes. I have some other branches to push related to code cleaning up. I will open other PRs in the next couple of weeks

I have some initial comments from inspection, and I will move to test this later this morning. That said, it feels like it might be worth splitting this PR up a bit so we can tackle more specific and focused testing.

@daledupreez I tried to make this PR focused on just removing the payment method classes, to avoid it getting too big or risky. Which parts do you think would be better to move to another PR?

[daledupreez]

For splitting the PR, I was thinking it might make sense to change the references to WC_Gateway_Stripe_Sepa::ID (which occurs a few times) to 'stripe_sepa' (or another constant), as changing the string and what we reference can happen without removing the underlying class. It also touches quite a few files, but is pretty easy to validate from inspection alone, as this should be an implementation detail.

[wjrosa]

Oh right. I created the other PR here: #4802