Drop output changing uniqueID calls, fix for MR !69737

HepMC2 GenVertex has a barcode() method (returns a manually set unique identifier) and an id() method which was used to hold the "status" of the GenVertex (basically indicated the process which caused the decay/interaction represented by the vertex).
HepMC3 GenVertex has an id() method (returns an automatically set unique identifier) and a status() method which is used to hold the "status" of the GenVertex().
xAOD::TruthVertex is based on HepMC2 GenVertex, so has corresponding barcode() and id() methods. In !67195 we are attempting to migrate xAOD::TruthVertex to be more like HepMC3::GenVertex.

While working on !67195 and !71935 (merged) it was noticed that the HepMC::uniqueID() helper function was incorrectly being used with xAOD::TruthVertex* in a few places.

template <class T> inline int uniqueID(const T& p) { return p->id(); }

This function is intended to work with the HepMC3 GenParticle/Vertex::id() (a unique identifier). xAOD::TruthParticle has an id() method which returns a unique identifier (in the form of the barcode()), however as xAOD::TruthVertex is still based on the HepMC2 GenVertex class its id() method, returns a (non-unique) value equivalent to status() rather than a unique identifier. Currently it is best to use xAOD::TruthVertex::barcode() via the HepMC::barcode() helper function when a unique identifier is required for instances of xAOD::TruthVertex.

It is anticipated that this merge request will change the barcode values for some entries in some xAOD::TruthVertexAuxContainer instances.

The issue was introduced in !69737 (merged).