Deprecating the UPE preview feature flag and removing its usages

[wjrosa]

Changes proposed in this Pull Request:

In this PR, I am deprecating the is_upe_preview_enabled method and removing the actual feature flag (_wcstripe_feature_upe). This has not been used for a while, since we deprecated the legacy checkout. I am also updating all parts of the code referencing the is_upe_preview_enabled method.

Testing instructions

Code review. Check if the tests are still passing. Perform some basic smoke-testing.

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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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