chore(checkout): CHECKOUT-10115 Remove rolled out CHECKOUT-10115 flag

[bc-maxy]

What/Why?

The flag is now fully rolled out across all tiers: https://app.launchdarkly.com/projects/default/flags/CHECKOUT-10115.consistent_crossorigin_prefetch_fix so we can remove the references from checkout-js.

It is passed from bigcommerce side through a variable, see here: https://github.com/bigcommerce/bigcommerce/blob/149504273c74b1b175e0c9e02929c1d825b239b0/config/Experiments/CheckoutExperiments.php#L61

Rollout/Rollback

Revert the PR

Testing

Passing CI, functional tests

---

Note

Low Risk
Cleanup only; prefetch crossorigin behavior was already the default via the rolled-out flag. Revert restores the flag gate if needed.

Overview
Removes the fully rolled-out CHECKOUT-10115 experiment flag (isConsistentCrossOriginFixEnabled) from checkout-js after LaunchDarkly rollout.

Behavior is unchanged for production: dynamic JS/CSS chunk prefetches in loadFiles now always use crossOrigin: 'anonymous', matching what the flag already enforced everywhere. The option is dropped from LoadFilesOptions, checkoutConfig on the custom checkout window, and the auto-loader no longer reads or forwards it.

Tests are simplified so prefetch expectations always include crossOrigin: 'anonymous' instead of duplicating cases behind the flag.

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