Remove unnecessary css for .documentation-index

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/assets/stylesheets/pages/help.scss

-.documentation-index {
-  li {
-    line-height: 24px;
-    color: $document-index-color;
-
-    a {
-      margin-right: 3px;
-    }
-  }
-}
-
-
 .shortcut-mappings {
   font-size: 12px;
   color: $help-shortcut-mapping-color;