ProjectExplorer: Remove some unused BuildManager signals

Change-Id: Iea832e96b726db9b93c235a57d6285337df7734d
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/projectexplorer/taskwindow.cpp

@@ -380,7 +380,6 @@ void TaskWindow::clearTasks(Core::Id categoryId)
     d->m_model->clearTasks(categoryId);
 
     emit tasksChanged();
-    emit tasksCleared();
     navigateStateChanged();
 }