disabled creation of PixelFEDChannelCollection by MixingModule in pre…

…mix stage2

The PixelFEDChannelCollection produced by the MixingModule in premix stage2
is not used at all so there is no point is creating it. Another
PixelFEDChannelCollection is produced later by the DataMixer and propagated
to tracking (after undergoing DigiToRaw and RawToDigi steps).

SimGeneral/MixingModule/python/pixelDigitizer_cfi.py

@@ -20,7 +20,8 @@
 # customization applies only to phase0/1 pixel.
 from Configuration.ProcessModifiers.premix_stage2_cff import premix_stage2
 premix_stage2.toModify(pixelDigitizer,
-    AddPixelInefficiency = False # will be added in DataMixer
+    AddPixelInefficiency = False, # will be added in DataMixer
+    KillBadFEDChannels = False # will be added in DataMixer
 )
 
 from SimTracker.SiPhase2Digitizer.phase2TrackerDigitizer_cfi import phase2TrackerDigitizer as _phase2TrackerDigitizer, _premixStage1ModifyDict