Remove swt-simple setting from CSS

simple is no-opt since
eclipse-platform/eclipse.platform.swt#3168

Author: vogella

bundles/org.eclipse.ui.themes/css/e4_default_gtk.css

@@ -112,10 +112,6 @@ ColorDefinition#org-eclipse-ui-workbench-INACTIVE_TAB_TEXT_COLOR {
 	background-color: COLOR-WIDGET-BACKGROUND #f6f5f4 100%;
 }
 
-.MPartStack {
-	swt-simple: false;
-}
-
 CTabFolder.MArea {
 	background-color: '#org-eclipse-ui-workbench-SECONDARY_BACKGROUND';
 	swt-selected-tab-fill: '#org-eclipse-ui-workbench-SECONDARY_BACKGROUND';