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].

Reviewed by audrasjb, SergeyBiryukov.
Merges [62377] to the 7.0 branch.

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

git-svn-id: https://develop.svn.wordpress.org/branches/7.0@62381 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

src/wp-admin/css/edit.css

@@ -5,7 +5,6 @@
 
 #poststuff #post-body {
 	padding: 0;
-	margin: 0 -4px;
 }
 
 #poststuff .postbox-container {