Revert PFElecTkProducer to legacy module because of TMVA

The TMVA code is not thread safe so PFElecTkProducer which uses this
must be reverted back to a legacy module.
Merge pull request #3918 from Dr15Jones/revertPFElecTkProducerToLegacy

RecoParticleFlow/PFTracking -- Revert PFElecTkProducer to legacy module because of TMVA
Merge remote branch 'origin/CMSSW_7_1_X' into CMSSW_7_0_X