DolphinQt/GameConfigEdit: Remove unimplemented SetReadOnly() member function

This isn't implemented anywhere, so we can remove this prototype.

Source/Core/DolphinQt/Config/GameConfigEdit.h

@@ -34,8 +34,6 @@ private:
   void OnAutoComplete(const QString& completion);
   void OpenExternalEditor();
 
-  void SetReadOnly(bool read_only);
-
   QString GetTextUnderCursor();
 
   void AddBoolOption(QMenu* menu, const QString& name, const QString& section, const QString& key);