Replaced pixelVertices by firstStepPrimaryVertices made from iter0 tracks

Remove pixelTracks and pixelVertices from reco sequence.
Replaced with the higher quality firstStepPrimaryVertices introduced together with JetCoreTracks in 7.2

Minor regression observed.
One may even pretend that fake-rate decreases and conversions benefit.

@VinInn 
Hi Vincenzo

It seems you define here, for full tracking, you define the vertex collection explicitly, 
while you don't do it in FastSimulation/Tracking/python/IterativeInitialStep_cff.py.
Is that propper?

Thanks

Lukas