Drop unused concept of shared child particle barcodes

A long time ago, there was the idea that all child particles produced in a decay could be assigned the same barcode. This schema was never used and now we are migrating away from barcodes generally it seems like a good time to clean this unused code up.

• TruthSvc: Drop setSharedChildParticleBarcode method
• IBarcodeSvc + implementations: Drop sharedChildBarcode method
• ITruthIncident + implementations: Drop setAllChildrenBarcodes method
• ISFParticle: Drop unused setBarcodeAndUpdateHepMcParticleLink method (this was only called by ISFTruthIncident::setAllChildrenBarcodes)