Core: remove unused member

Change-Id: I3088055bd395a432b0da8d76b9179944c9a2ad80
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/coreplugin/idocumentfactory.h

@@ -59,7 +59,6 @@ public:
     void addMimeType(const QString &mimeType) { m_mimeTypes.append(mimeType); }
 
 private:
-    Utils::Id m_id;
     Opener m_opener;
     QStringList m_mimeTypes;
     QString m_displayName;