Fix Pythia8 OTF weights for HepMC3
As mentioned in ATEAM-869, we have inconsistent weight vectors. These originate from a mis-match between what is being stored into GenEvent in the Pythia8 HepMC3 convener and what is expected in Pythia8_i. I this MR, we disable the storing of these weights in the Pythia8 HepMC3 converter and instead rely on our Pythia8_i to set these.
Edited by Matthew Gignac