ManhattanStyle: Remove unused special property

That was used during the times where OS X still had alternating row
colors in item views.

Change-Id: I6622604502a768c4abeb42f25bcfea5c65f66395
Reviewed-by: Daniel Teske <daniel.teske@theqtcompany.com>

src/plugins/coreplugin/manhattanstyle.cpp

@@ -354,11 +354,6 @@ int ManhattanStyle::styleHint(StyleHint hint, const QStyleOption *option, const
 {
     int ret = QProxyStyle::styleHint(hint, option, widget, returnData);
     switch (hint) {
-    // Make project explorer alternate rows all the way
-    case QStyle::SH_ItemView_PaintAlternatingRowColorsForEmptyArea:
-        if (widget && widget->property("AlternateEmpty").toBool())
-            ret = true;
-        break;
     case QStyle::SH_EtchDisabledText:
         if (panelWidget(widget) || qobject_cast<const QMenu *> (widget) )
             ret = false;