purge unstable particles from Hijing

Occasionally, Hijing produces unstable particles (i.e. with status=2) without end vertex. That is disliked by simulation that throws errors because of that.

It was attempted to fix it by filtering such particles, see !65417 (merged). Unfortunately, FixHepMC can also delete particles, and if it removes the last outgoing particle from the vertex, it also deletes the vertex. This leaves the mother particle without the end vertex, leading to problems in simulation.

This MR moves the filtering to FixHepMC, removing all particles without end vertex regardless if created by Hijing or by FixHepMC. It also switche this filtering on for Hijing; behaviour of all other generators is unchanged.

tagging @stapiaar @ewelina @tbold

related JIRA ticket: ATLHI-483