ProjectManager: Remove SessionNode

Instead, get root project nodes directly from the project.

Change-Id: I5cf95a7dce1fa29c1adc26013ad03cc017f38a6d
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/plugins/projectexplorer/projectwizardpage.h

@@ -95,7 +95,7 @@ private:
 
     Ui::WizardPage *m_ui;
     QStringList m_projectToolTips;
-    Utils::TreeModel<> *m_model = nullptr;
+    Utils::TreeModel<> m_model;
 
     QList<Core::IVersionControl*> m_activeVersionControls;
     QString m_commonDirectory;