FSE: clean up unused code after the rollout

[vindl]

The purpose of this issue is to track some placeholder code that we can clean up after FSE is fully rolled out:

• [x] Remove is_core_fse_active checks and replace usages with is_fse_theme. Follow up to this PR: https://github.com/Automattic/wp-calypso/pull/61040 - (WPCOM PR for removal: D83298-code)
• [x] Remove the fse-eligible sticker toggle in the Blog RC. (WPCOM PR for removal: D83050-code)
• [x] Update the FSE activation section and indicators in Blog RC. (WPCOM PR for update: D83340-code)
• [ ] Update the FSE Field Guide documentation.
• [ ] Update FSE support Docs if necessary.
• [x] Remove is_fse_eligible related conditionals in calypso, default to fse eligible behavior. (Calpyso PR for removal: https://github.com/Automattic/wp-calypso/pull/64710)
• [ ] Remove is_fse_eligible field from block editor endpoint, and wpcom_is_fse_eligible function and its remaining uses.
• [x] Remove site option functions from ETK, and their uses across tests and site creation hooks.
• [x] Deprecate calypso_fse_enrolled event and stop recording it. (Calypso PR for removal: https://github.com/Automattic/wp-calypso/pull/64710)
• [x] Deprecate calypso_fse_beta_opt_in event.
• [x] Clean up non-universal-fse tag and usages.
• [ ] Remove the existing is_core_site_editor_enabled property and related code from the v1.1 /sites endpoint

After the plan to migrate legacy FSE sites to Core FSE is executed (see pbAok1-27H-p2), there is a lot of code that we can remove as it will no longer be used by any sites on WP.com. It's important to do this, as leaving this code in place creates confusion because it refers to "FSE" but not the features and functionality that most people now think of when seeing FSE.

This list likely incomplete, but here's a start to what we should remove/change

• [x] Remove dotcom-fse code from the ETK plugin and wpcom (https://github.com/Automattic/wp-calypso/tree/trunk/apps/editing-toolkit/editing-toolkit-plugin/dotcom-fse)
• [x] Remove \A8C\MU\Full_Site_Editing\Full_Site_Editing file and class from wpcom
• [x] Remove namespace A8C\MU\Full_Site_Editing from helper functions (enroll_in_fse and is_fse_eligible)
• [ ] Ideally it would be good to get rid of the singleton classes in mu-plugins/full-site-editing, as these are mainly being used to namespace functions that already have PHP namespaces 🙃
• [x] Remove unnecessary submenu hiding of "Styles" entry point (https://github.com/Automattic/wp-calypso/issues/57997)

[Addison-Stavlo]

adding https://github.com/Automattic/wp-calypso/issues/57997 to the list above.

[creativecoder]

Regarding the fse-eligible sticker... there is also some related code to remove in both Calypso and wpcom:

Calypso

I think it's just this one line, sending the enable_fse parameter

https://github.com/Automattic/wp-calypso/blob/e4a70f5af0d7c5d1792403c3691585d587f03c5a/client/lib/signup/step-actions/index.js#L179

wpcom

I believe the whole wp-content/mu-plugins/full-site-editing/class-site-editor.php file can be removed, as we don't need to enroll new sites any longer. Additionally the helper function wpcom_enroll_in_fse() can be removed, as that's the only place it's used.