SessionManager: Remove declaration of not implemented function

The function was removed long ago.

Change-Id: Iee1e413bb675e360eb15a89bcb5d37ac83b40e84
Reviewed-by: Tobias Hunger <tobias.hunger@nokia.com>

src/plugins/projectexplorer/session.h

@@ -117,7 +117,6 @@ public:
 
     // NBS rewrite projectOrder (dependency management)
     QList<Project *> projectOrder(Project *project = 0) const;
-    QAbstractItemModel *model(const QString &modelId) const;
 
     SessionNode *sessionNode() const;