Fix for double usage of the seeding track in IVF

Fix for double usage of the seeding track in IVF. Impact on the b-tagging performance small as can be seen from the attached plot (red with fix, black without).

![image](https://cloud.githubusercontent.com/assets/4885289/2910017/44e64908-d63b-11e3-9ffb-326417ecac50.png)