Merge pull request #721 from hypothesis/remove-unused-scss

Remove used used scss block causing invalid css error after upgrading…

src/styles/sidebar/sidebar.scss

@@ -70,10 +70,6 @@ hypothesis-app {
   }
 }
 
-#{nest("hgroup", "#{headings()}")} {
-  margin: 0;
-}
-
 .content {
   margin-left: auto;
   margin-right: auto;