Products Onboarding: Remove Unnecessary code

[Ecarrion]

Description

We borrowed some code from the WP app in order to self-authenticate reviews to be able to load draft previews. Later we found that we could achieve the same with a frame nonce.

As that solution is working well enough, we should delete the borrowed code we introduced before.

[peril-woocommerce]

	Fails
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🚫	Please add a feature label to this issue. e.g. 'feature: stats'

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