ActionBarSherlock stacked action bar styling issue
You have to change the android:actionBarDivider attribute which belongs to the theme, not to the action bar style android:divider. You can remove the divider like this:
<style name="AppTheme" parent="Theme.Sherlock">
<item name="actionBarDivider">@null</item>
<item name="android:actionBarDivider">@null</item>
</style>