Remove mobile editor preference on Simple sites

[guarani]

Description

There is a mobile editor preference switch ("Use block editor" toggle switch in My Site → Settings) that allows site administrators to make the classic editor the default for all new content on that site. This issue tracks the need to remove that switch on Simple WP.com sites, and by doing so, make the block editor the default on this type of site.
The exception to this is for the user flow where a post is created via the WordPress share extension, which will continue to use the classic editor.

To see the toggle switch

1. Log into the app
2. Choose a Simple WP.com site you're an admin of
3. Go to My Site → Settings and notice that there is a "Use block editor" toggle switch

What will change

1. Site → Settings should no longer contain a "Use block editor" toggle switch
2. New posts/pages should always use the block editor (except those created via the WPiOS share extension)

Screenshots

Smartphone (please complete the following information):

• Device: Android or iOS
• OS: any version
• Version WP 16.5

[guarani]

Note that within the block editor, there will still be the option to switch to the classic editor.

I removed this line from the issue since this "switch to classic" option will be removed in #3048.

[guarani]

I've updated this issue to be Simple site specific and created a separate issue for Atomic sites since these will be shipped separately.

[guarani]

This was done in wordpress-mobile/WordPress-iOS#15766 and wordpress-mobile/WordPress-Android#14381.