Revert "MainWindow: Delete navigationwidget again"

This causes trouble with the unit tests on windows.

This reverts commit 0fe28d652f1219978d23030cbb322d09ada5a68f

Change-Id: I08d4991d9b86ee8b80d093899f2e33e2be1c1565
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>

src/plugins/coreplugin/mainwindow.cpp

@@ -306,9 +306,6 @@ MainWindow::~MainWindow()
     m_helpManager = 0;
     delete m_variableManager;
     m_variableManager = 0;
-
-    delete m_navigationWidget;
-    m_navigationWidget = 0;
 }
 
 bool MainWindow::init(QString *errorMessage)