Delete more unused WordPressKit and WordPressAuthenticator code

[mokagio]

Builds on top of #14779, see rationale in #14329

The same consideration as #14790 apply: given this is a deletion only PR meant to slim #14779, I'll admin merge it as soon as CI allows it.

[dangermattic]

	1 Warning
⚠️	This PR is larger than 300 lines of changes. Please consider splitting it into smaller PRs for easier and faster reviews.

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[wpmobilebot]

📲 You can test the changes from this Pull Request in WooCommerce iOS by scanning the QR code below to install the corresponding build.

	App Name	WooCommerce iOS
	Build Number	pr14794-f12f512
	Version	21.3
	Bundle ID	com.automattic.alpha.woocommerce
	Commit	f12f512
	App Center Build	WooCommerce - Prototype Builds #12353

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