TaskModel: Do not duplicate items

When inserting new items into the TaskFilterModel we could end up
duplicating the items. Do not do that.

Change-Id: I2450bac185a881e16f23368b39ce5e273c061d99
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>

src/plugins/projectexplorer/taskmodel.h

@@ -179,7 +179,6 @@ private:
     QList<Core::Id> m_categoryIds;
 
     mutable QList<int> m_mapping;
-    mutable bool m_mappingUpToDate;
 
     TaskModel *m_sourceModel;
 };