Update removed "menu-icon" class from all list items.

Version 2.2 changed the line that adds the menu-item class to <li> tags, from:

if (!empty($link_plugin['options']['menu_icon']['fid'])) {
  $mlid = $link_plugin['metadata']['entity_id'];
  $item['attributes']->addClass('menu-icon menu-icon-' . $mlid);
}

to:

$mlid = $link_plugin['metadata']['entity_id'];
$item['attributes']->addClass('menu-icon-' . $mlid);

This broke my site, leaving me without a way to apply a uniform style all menu icon items (I used this to hide the text while still making it accessable to screen readers). Perhaps there's a reason this was removed that I don't understand, since the if block that surrounds those lines that was also removed, but it's possible this was a simple oversight, since there was also a lot of refactoring in that file.