Remove option to use classic editor (unless classic editor plugin enabled)

[adamsilverstein]

Describe the bug
After upgrading to WordPress 5.0 and installing the plugin, I am presented with the option to edit content in the classic editor even though I don't have the classic editor plugin installed.

Related: #11015

[designsimply]

Tested and confirmed that I see the option to edit content in the classic editor withe the Gutenberg 4.7.0 plugin installed on WordPress 5.0.1.

If I click the "Classic Editor" link on the Posts or Pages screen and then click Continue, I see a blank screen and the following error in the console:

TypeError: window._wpLoadBlockEditor is undefined
post.php:1731:1

^{Seen at https://testingallthetime.com/wp-admin/post.php?post=47&action=edit&classic-editor running WordPress 5.0.1 and Gutenberg 4.7.0 using Firefox 63.0.3 on macOS 10.13.6.}

[nfmohit]

Submitted #13217 addressing this issue.

[aduth]

This code should probably be removed altogether.

I'd started down this path on Friday, but there's a fair bit of the metabox code which makes assumption about ?classic-editor. In fact, most of this code ought to probably also be removed, with an assumption that it be provided already through WP 5.0+ (set as a new minimum required version for the Gutenberg plugin).

[youknowriad]

I think this one done recently right @aduth ?

[aduth]

It's partially resolved by #13459, with final pieces slated to be removed by #13544 (approved).

I'll add a closing reference accordingly.