PluginView: remove stale includes

Change-Id: Ic3484d8694b54032c8ee3dbb9aa45b97e19c9b6c
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/libs/extensionsystem/pluginview.h

@@ -30,18 +30,11 @@
 #include <utils/treemodel.h>
 
 #include <QWidget>
-#include <QSet>
-#include <QHash>
-
-QT_BEGIN_NAMESPACE
-class QSortFilterProxyModel;
-QT_END_NAMESPACE
 
 namespace Utils { class TreeView; }
 
 namespace ExtensionSystem {
 
-class PluginManager;
 class PluginSpec;
 
 namespace Internal {