fix vertex loop in PFPhotonIsolationCalculator

Removed an "obfuscated" nested loop.
also profit of the new template interface to remove RefBase churn...
(still, it is looping on all the tracks in all the vertices for each charged particle...)

accounting for more than 1% of the time in Reco 40PU25ns TTBAR
http://innocent.web.cern.ch/innocent/perfResults/igprof-navigator/TTBAR_step3_ori_CMSSW_7_1_X_2014-04-05-1400_slc6_amd64_gcc490/190

No regression expected. No regression observed