Remove the switch to Classic editor from within the Block editor on Atomic WP.com sites

[guarani]

#3092 tracks the need to remove the "Use block editor" switch from site settings. This issue addresses the need to remove the "Switch to classic editor" option from the block editor itself as well, for Atomic sites.

Describe the solution you'd like

The above mentioned issues discuss which site types are affected by the site settings switch removal change. The site types affected by this change will likely be the same or similar. The screenshot below shows the setting which we're considering removing.

In WP Admin, the setting that determines whether or not users can switch to the classic editor while using the Block editor is called "Allow users to switch editors" and is found under Setting → Writing for sites that have the Classic plugin activated. It is not available in Calypso, AFAIK.
When this is ON, the editor toggle setting can be found in the editor's ellipsis menu, under Plugins, and is called "Switch to classic editor".

Describe alternatives you've considered

We could delay removing this setting and wait for more Core blocks to be supported – however, the block editor supports the most commonly used blocks and is more feature-packed than the classic editor, so it feels like now is the right time to make this change.

[guarani]

Closing in favor of #3048 since we decided to remove the "switch to classic" option for all site types at once.