WPDS token fallbacks: clean out various screens (SEO, Newsletter, Scan)

[simison]

Follow up to PRs:

• Stylelint: no WPDS token fallback values for WP Build routes #50502
• Newsletters: add WPDS fallbacks during build #50583
• Stylelint: no WPDS token fallback values for WP Build routes #50502

Proposed changes

• Removes token fallbacks from various screens (SEO, Newsletter, Scan)
• Adds folders to "no fallbacks" lint rule list

These are already built by pipelines which add fallbacks, so there's no difference in output bundles.

There were a few incorrect fallbacks already, now corrected:

Token	Was	Official
--wpds-color-foreground-content-neutral-weak	#757575, #4d5156, #787c82	#707070
--wpds-color-background-surface-neutral	#f6f7f7, #f0f2f5	#fcfcfc
--wpds-color-stroke-surface-neutral-weak	#dadde1, #e0e0e0	#f0f0f0

Related product discussion/links

Does this pull request change what data or activity we track or use?

Testing instructions

• Smoke test relevant UIs.
• Confirm bundles stay the same apart from incorrect fallbacks now corrected

[github-actions]

Are you an Automattician? Please test your changes on all WordPress.com environments to help mitigate accidental explosions.

• To test on WoA, go to the Plugins menu on a WoA dev site. Click on the "Upload" button and follow the upgrade flow to be able to upload, install, and activate the Jetpack Beta plugin. Once the plugin is active, go to Jetpack > Jetpack Beta, select your plugin (Jetpack), and enable the update/wpds-fallbacks-leftovers branch.
• To test on Simple, run the following command on your sandbox:

bin/jetpack-downloader test jetpack update/wpds-fallbacks-leftovers

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[github-actions]

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Code Coverage Summary

This PR did not change code coverage!

That could be good or bad, depending on the situation. Everything covered before, and still is? Great! Nothing was covered before? Not so great. 🤷

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