CppEditor: Make a few CppModelManager functions static

... to simplify code using it.

Change-Id: Ib2bfbb5a17a1d48088a9cf4973d4c3f006cd34e4
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/cppeditor/cpprefactoringchanges.h

@@ -76,7 +76,6 @@ public:
     void fileChanged(const Utils::FilePath &filePath) override;
 
     CPlusPlus::Snapshot m_snapshot;
-    CppModelManager *m_modelManager;
     WorkingCopy m_workingCopy;
 };