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/autotoolsprojectmanager/autotoolsproject.cpp

@@ -229,6 +229,7 @@ void AutotoolsProject::makefileParsingFinished()
                              f == QLatin1String("configure.ac")) ? FileType::Project : FileType::Resource,
                             false);
     });
+    rootProjectNode()->makeEmpty();
     rootProjectNode()->buildTree(fileNodes);
 
     updateCppCodeModel();