IEditor: Make duplicatesSupported a value member

Change-Id: Iee75dd4155d5074bb319df79d006b6d5e2989e14
Reviewed-by: Eike Ziller <eike.ziller@digia.com>

src/plugins/diffeditor/diffeditor.h

@@ -62,7 +62,6 @@ public:
     DiffEditorController *controller() const;
 
     // Core::IEditor
-    bool duplicateSupported() const { return false; }
     Core::IEditor *duplicate();
 
     bool open(QString *errorString, const QString &fileName, const QString &realFileName);