[calamares] Silence QBasicTimer warning message.

Relying on auto-qobject deletion on shutdown generates a warning message
    17:23:44 [0]: QBasicTimer::start: QBasicTimer can only be used
                  with threads started with QThread
which is annoying. Since we're in shutdown, just skip deletion
entirely, and leave the model unparented.

FIXES #472

src/calamares/CalamaresApplication.cpp

@@ -356,7 +356,7 @@ CalamaresApplication::initViewSteps()
 {
     cDebug() << "STARTUP: loadModules for all modules done";
     m_mainwindow->show();
-    ProgressTreeModel* m = new ProgressTreeModel( this );
+    ProgressTreeModel* m = new ProgressTreeModel( nullptr );
     ProgressTreeView::instance()->setModel( m );
     cDebug() << "STARTUP: Window now visible and ProgressTreeView populated";
 }

src/calamares/progresstree/ProgressTreeView.cpp

@@ -62,7 +62,6 @@ ProgressTreeView::ProgressTreeView( QWidget* parent )
 
 ProgressTreeView::~ProgressTreeView()
 {
-
 }