Remove duplicated GenEvent vertex filling for truth pileup config

In the context of restoring some functional full-truth pile-up configuration (see ATLSWUPGR-167), the current config stores duplicated pile-up TruthParticles in the AOD. Following up !60224 (merged), the GenEvent already inherits from the vertices from the original background event and adding it again below leads to some duplication. The updateClassificationMap is still needed though. This will likely be revisited with a future reworking of the MCTruthSimAlgs undertaken by @jchapman but it was checked to provide a relevant fix for full-truth tracking studies, which is a desirable feature for short-term upgrade studies.

This change doesn't affect default StandardSignalTruth digi config.

FYI @jchapman @tadej @nstyles @dshope @jmasik @pagessin @lgagnon