[FIX] web_editor: error when long delete keypress in mass_mailing html

issue: event does not a global var in firefox, and event is already prevented

addons/web_editor/static/src/js/summernote.js

@@ -1226,7 +1226,6 @@ function remove_table_content(r) {
                 node.parentNode && !$(node.parentNode).hasClass('o_editable'));
         }
     }
-    event.preventDefault();
     return false;
 }