Remove Lefovers from the "Classic" Widget

[cabrerahector]

After the official removal of the "classic" widget we still need to work on the following:

• Remove the /wp-json/wordpress-popular-posts/v1/popular-posts/widget/ endpoint.
• Remove the WordPressPopularPosts\Widget class.
• Remove related JS code from /assets/js/wpp.js.
• Remove related CSS selector from Front.php (see inline_loading_css()).
• Update all widget related entries on the wiki (including REST API endpoint docs).