HepMcParticleLink Drop support for separate McEventCollections for pile-up truth

The idea of having separate McEventCollection instances for each type of pile-up used was never used in production and with the ability to decorate GenEvents directly with this information using Attributes is now completely redundant.

Removing this will simplify the HepMcParticleLink EDM. Unfortunately due to the large number of client classes then this merge request is quite large. Here is a breakdown to ease the review process:

1. The main changes are in the Generators/GeneratorObjects package. Removing the obsolete code and in particular revising the ExtendedBarCode and HepMcParticleLink constructor syntax.
2. The next set of changes are in MuonSpectrometer/MuonPhaseII/Event/xAOD/xAODMuonSimHit/Root/xAODMuonSimHit_V1.cxx and InnerDetector/InDetEventCnv/InDetSimEventTPCnv/src/InDetHits/*.cxx hard-coding variables in some persistent class objects to the only value ever used in production. (Future versions of these persistent classes will drop this variable, but that will be a separate merge request.)
3. The next set of changes are in the various TP converter test classes - avoiding checks on the variable(s) now removed from the transient HepMcParticlelink class.
4. The rest of the changes are dropping the obsolete arguments from HepMcParticleLink constructor calls.

Tagging @ewelina, @averbyts, @pclark