TinyMCE: improve the previews for embedded WordPress posts:

- Add option to force a sandbox iframe in wpview.
- Use it to show the embedded post preview.
- Remove the deprecated `wpembed` plugin.js

Fixes #39513.

git-svn-id: https://develop.svn.wordpress.org/trunk@40019 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp-editor.php

@@ -417,7 +417,6 @@ final class _WP_Editors {
 						'wpdialogs',
 						'wptextpattern',
 						'wpview',
-						'wpembed',
 					);
 
 					if ( ! self::$has_medialib ) {