74X fixes to CSC Digitization/Trigger paths for PreMixing

[mdhildreth]

cleanup to drop error messages in PreMixing Step, proper treatment of CSC digis now that the MC Packer is run by default.

[mdhildreth]

+1

[cmsbuild]

A new Pull Request was created by @mdhildreth for CMSSW_7_4_X.

74X fixes to CSC Digitization/Trigger paths for PreMixing

It involves the following packages:

Configuration/PyReleaseValidation
SimGeneral/DataMixingModule

@cmsbuild, @srimanob, @nclopezo, @boudoul, @franzoni can you please review it and eventually sign? Thanks.
@ghellwig, @Martin-Grunewald, @wmtan this is something you requested to watch as well.
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[davidlange6]

@mdhildreth - this means we can remove the premix customize functions altogether? (for the next PR)
Given that RECO with premixing is not functioning,I'll bypass this

[mdhildreth]

Unless they are not used at all by default, you still need the PostLS1PreMixing customize for the Digi step (step2), since it gives the trigger the correct CSC inputs.