Session: Update SessionNode when Project tree changes

This moves ownership of the project's root node from the
project into the project tree!

Change-Id: I84eba884bd63b44e56c75023d8bf12caf5cc2833
Reviewed-by: hjk <hjk@qt.io>

src/plugins/cmakeprojectmanager/cmakeproject.cpp

@@ -134,7 +134,6 @@ CMakeProject::~CMakeProject()
         future.waitForFinished();
     }
     delete m_cppCodeModelUpdater;
-    setRootProjectNode(nullptr);
     qDeleteAll(m_extraCompilers);
     qDeleteAll(m_allFiles);
 }