Updated PU check for TruthClusterizationFactory [ATLITKSW-216]

Now that some reasonable full-truth config has been identified, the TruthClusterizationFactory implementation has been investigated with a full-truth sample. It turns out that the eventIndex check simply needs to be updated to check the actual eventNumber from the EventInfo. In case of HepMcParticle from PU, this check will fail and they can be discarded based on this. The tweak in the configuration can thus be dropped. Fix ATLITKSW-216

FYI @pagessin @lgagnon