Fix warning in RecoParticleFlow/PFClusterProducer: Removed sortByDepth since it was not being used

This fixes the the warning that can be seen here: https://cmssdt.cern.ch/SDT/cgi-bin/buildlogs/slc6_amd64_gcc491/CMSSW_7_2_X_2014-08-12-0200/RecoParticleFlow/PFClusterProducer