CppTools: Use unshared object for CppCodeModelSettings

There's nothing shared here, only ever one copy. Ideally, this
should be const outside CppToolsPlugin, but some settings
are modified directly.

Change-Id: I775b9151a244b3cc44d28bc992a041c42d234a18
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/cpptools/cpptoolsplugin.h

@@ -70,7 +70,7 @@ public:
     bool initialize(const QStringList &arguments, QString *errorMessage) final;
     void extensionsInitialized() final;
 
-    QSharedPointer<CppCodeModelSettings> codeModelSettings() const;
+    CppCodeModelSettings *codeModelSettings();
 
 public slots:
     void switchHeaderSource();