Switch to using the assignment operator of xAOD::TruthParticle

MergeTruthParticlesTool and CopyPileupParticleTruthInfo previous copied xAOD::TruthParticle objects by-hand, but this approach meant that these classes needed to be updated any time decorations were changed. Switching to using the assignment operator in these classes makes the code simpler to read and more robust against future changes. No changes in output observed.