InDetSegmentDriftCircleAssValidation: HepMC3 fixes for HepMcParticleLink.
For HepMC3, HepMcParticleLink should yield a ConstGenParticlePtr rather than a bare pointer.
Also remove some unused uses of GenParticle*.
For HepMC3, HepMcParticleLink should yield a ConstGenParticlePtr rather than a bare pointer.
Also remove some unused uses of GenParticle*.