Remove newsletter signup card AB test

[georgerichmond]

What does this change?

• Deletes the newsletters-signup-card-country-illustration A/B test entry from abTests.ts, freeing up space B
• Changes the fallback in getVariantName() from 'control' to 'variantIllustratedCard', so all eligible web users now receive the illustrated card regardless of AB participation
• Updates the test suite to reflect the new behaviour — removes the dead variantNewField test cases and updates assertions to expect newsletter-signup-card-container for all web users with the flag on

The AB framework itself (useAB, abResolved gating, the placeholder while hydrating) is left untouched, ready for a future test.

Why?

The variantIllustratedCard design has been validated has been set 100% of users. We need to free up the ab test slot for the Feast team. Rather than a large cleanup PR, this is the smallest safe step: delete the test and hardcode the winning variant as the default. A follow-up PR can remove the now-redundant AB resolution logic and dead code.

Screenshots

No visual change — users who were already in the variantIllustratedCard group see no difference, and users previously in control will now see the illustrated card for the first time.

Before	After

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[github-actions]

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From guardian/actions-riff-raff.