Editor: Remove an unwanted negative margin in Classic Editor metaboxes CSS styles.

This changeset reverts a negative `-4px` margin on `#poststuff #post-body` introduced in [61646].

Follow-up to [61646], [61759], [62284].

Props TobiasBg, umeshnevase, audrasjb, jadavsanjay, khokansardar, darshitrajyaguru97, gaurangsondagar.
Fixes #65141.



git-svn-id: https://develop.svn.wordpress.org/trunk@62377 602fd350-edb4-49c9-b593-d223f7449a82