Remove unused private functions

As found by Cppcheck.

Change-Id: I697ea69992425ff41e35990ad00be312b35ba70a
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/libs/cplusplus/CppDocument.h

@@ -293,9 +293,6 @@ public:
         { return _beginLine; }
 
     private:
-        void setArguments(const QVector<Block> &arguments)
-        { _arguments = arguments; }
-
         void addArgument(const Block &block)
         { _arguments.append(block); }