ProjectExplorer: Nodes: Do not makeEmpty() as part of buildTree()

Do not call makeEmpty() as part of buildTree(). That makes it
impossible to combine buildTree with manual tree setup.

Change-Id: If0a0d9432fe39870917a6ba31594e8dcd6d31868
Reviewed-by: hjk <hjk@qt.io>

src/plugins/nim/project/nimproject.cpp

@@ -159,6 +159,7 @@ void NimProject::updateProject()
     if (oldFiles == m_files)
         return;
 
+    rootProjectNode()->makeEmpty();
     rootProjectNode()->buildTree(fileNodes);
 
     emit fileListChanged();