Remove unused private member variable m_id

Change-Id: I23907f9d95e6abd72980391e082159e61f8fecc5
Reviewed-by: Eike Ziller <eike.ziller@digia.com>

src/plugins/coreplugin/editormanager/ieditor.h

@@ -63,9 +63,6 @@ public:
     virtual QWidget *toolBar() = 0;
 
     virtual bool isDesignModePreferred() const { return false; }
-
-private:
-    Core::Id m_id;
 };
 
 } // namespace Core