Block Themes: Default body margin is removed in the Site Editor, but not in the front end.

[kjellr]

Description

The default body margin appears to be removed in the editor, but not in the front end. Since full-width items go full-width in the editor, I'd expect that is the correct behavior.

cc @youknowriad since I think you've looked into this situation before.

Editor	Front End

Step-by-step reproduction instructions

1. Use the emptytheme.
2. View the front end, observe that there's a slim margin around the entire body.
3. Open the site editor, observe that this margin is not there. The inspector shows that it's being removed somewhere.

Environment info

Gutenberg trunk (Version 11.6.0)