Hardcoding ECE feature flag to be always enabled

[wjrosa]

Fixes STRIPE-733
See #4687 (comment)

Changes proposed in this Pull Request:

In this PR, I am hardcoding the Express Checkout feature flag to be always enabled. This will force ECE for all merchants, even those that, for some reason, changed the default flag value, making it safe to remove all code related to Payment Request Buttons.

Testing instructions

Code review is enough.

---

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

Changelog entry

• This Pull Request does not require a changelog entry. (Comment required below)

Changelog Entry Comment

Comment

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)

[Mayisha]

@wjrosa this feature flag check becomes unnecessary as we are removing the PRB frontend and backend code/files completely. IMO we can remove the is_stripe_ece_enabled function and the corresponding calls to it. With PRB gone, ECE would be the only available option so we don't need the feature flag check anymore.

[wjrosa]

I agree. However, that was to address @daledupreez's concerns in #4687 (comment). It is still possible for a merchant to manually change the feature flag value, as far as I am aware. Let me know if that's a wrong assumption (I hope so).

[Mayisha]

I agree. However, that was to address @daledupreez's concerns in #4687 (comment). It is still possible for a merchant to manually change the feature flag value, as far as I am aware. Let me know if that's a wrong assumption (I hope so).

cc @daledupreez for clarification. I think the feature flag should be deleted. Also ECE has been default on for UPE since the beginning of this year

[daledupreez]

I saw at least one merchant with the express checkout feature flag disabled during my support rotation in September - they were still using PRBs. 😬 I didn't know that was still possible, but it clearly was.

With that in mind, I think it's probably safest to stick with @wjrosa's approach for the short term.

[daledupreez]

Other than the changelog entry wording, this LGTM.

[Mayisha]

Code looks good to me. 👍

However, I still think removing the is_stripe_ece_enabled completey vs returning true from this function would behave the same if we remove all the PRB related files (as there is no fallback and ECE is the only option available) 🤔