Fix for "_iAssignedPartition ist NULL"-exception on removeChild()

When calling removeChild(), an "_iAssignedPartition ist NULL" is thrown
when the child does not have an _explicitPartition set. This can be
fixed by calling „this._pUpdateImplicitPartition(null);“ AFTER
„this._iSetScene(null);“, so that the _pImplicitPartition of the child
is not null also.