Remove unused includes of QFutureInterface

Change-Id: I70f5e842801b628c7f9ad4d433334ce04d4e648e
Reviewed-by: Eike Ziller <eike.ziller@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

src/plugins/diffeditor/unifieddiffeditorwidget.h

@@ -15,10 +15,6 @@ template <typename R>
 class Async;
 }
 
-QT_BEGIN_NAMESPACE
-class QFutureInterfaceBase;
-QT_END_NAMESPACE
-
 namespace DiffEditor {
 
 class ChunkData;