Fix for panels layout - "Omega: 12 Column Grid - Two Column 6-6 stacked"

Just a very minor thing, but the two grid-6 panels shouldn't contain alpha & omega classes. Puts the layout a bit out. Here is the replacement code with them removed...

<div class="panel-display omega-grid omega-12-twocol-6-6-stacked" <?php if (!empty($css_id)) { print "id=\"$css_id\""; } ?>>
  <div class="panel-panel grid-12">
    <div class="inside"><?php print $content['top']; ?></div>
  </div>
  <div class="panel-panel grid-6">
    <div class="inside"><?php print $content['left']; ?></div>
  </div>
  <div class="panel-panel grid-6">
    <div class="inside"><?php print $content['right']; ?></div>
  </div>
  <div class="panel-panel grid-12">
    <div class="inside"><?php print $content['bottom']; ?></div>
  </div>
</div>