Clean up Limit Global Styles logic

[mmtr]

Proposed Changes

Cleans up some technical debt from the logic that limits Global Styles:

• The theme_json_user filter is no longer used to block styles since WP.com no longer loads the Gutenberg version that triggered it
• Remove usages of _Gutenberg classes since they are not intended to be used externally.

Testing Instructions

• Apply these changes to your sandbox: install-plugin.sh editing-toolkit update/clean-up-limit-global-styles
• Go to https://wordpress.com/start and create a new free site
• Add the wpcom-limit-global-styles blog sticker to the new site from the Blog RC
• Sandbox the new site
• Go to Appearance > Editor
• Open the Global Styles sidebar
• Change some styles
• Click on the "Save" button
• Visit the frontend
• Make sure only the styles from the default theme variation are visible
• Make sure the launchbar contains a "Styles hidden" action

[github-actions]

Calypso Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-50904

Jetpack Cloud live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-50904&env=jetpack

[matticbot]

This PR modifies the release build for editing-toolkit

To test your changes on WordPress.com, run install-plugin.sh editing-toolkit update/clean-up-limit-global-styles on your sandbox.

To deploy your changes after merging, see the documentation: PCYsg-mMA-p2

[matticbot]

This PR does not affect the size of JS and CSS bundles shipped to the user's browser.

Generated by performance advisor bot at iscalypsofastyet.com.

[dsas]

Pushed a fix for the fatal, though maybe you prefer to just pass wp_get_theme().

Pushed a fix for the fatal

[mmtr]

Pushed a fix for the fatal, though maybe you prefer to just pass wp_get_theme().

Thanks for that! Didn't notice the argument was actually required. I pushed an extra update to use wp_get_theme instead, since I think that's more readable.

[dsas]

LGTM 👍