OverlayCopyAlgs: CopyMcEventCollection: only remove separator GenEvents in HepMC3-based builds
Some clean-up for CopyMcEventCollection
behaviour in HepMC2-based builds - follows !75716 (merged)
In HepMC3-based builds separator GenEvents are replaced by setting the appropriate GenEvent Attributes - this is not possible in HepMC2-based builds (Attributes aren't part of HepMC2), so the separator GenEvents should be retained.
Edited by John Derek Chapman