Revert "Sidebar container block: Allow themes to override top margin"

This reverts commit ed9879d.

Fixes WordPress/wporg-documentation-2022#94

mu-plugins/blocks/sidebar-container/postcss/style.pcss

@@ -102,6 +102,6 @@ main .wp-block-wporg-sidebar-container {
 }
 
 /* Set up the custom properties. These can be overridden by settings in theme.json. */
-:where(html) {
+:where(body) {
 	--wp--custom--wporg-sidebar-container--spacing--margin--top: var(--wp--preset--spacing--edge-space);
 }