[IMP] mass_mailing: remove negative margin on iframe

When the first snippet in a mail is for example a cover image,
the html2canvas library exports the image with a white line along the
top caused by the negative top margin on the iframe.

task-2856858

Part-of: odoo/odoo#91314

addons/mass_mailing/static/src/scss/mass_mailing.ui.scss

@@ -344,9 +344,4 @@ body.editor_enable.o_basic_theme.o_in_iframe {
     .note-editable {
         padding: 0;
     }
-
-    //this will display the editor wrapper and the snippets at the same level
-    .iframe-editor-wrapper {
-        margin-top: -2px;
-    }
 }