WIP: Removal of HepMC2

Removes all HepMC2 dependent parts from LHCb. As we are not supporting HepMC(3) output in the upgrade, these parts are not required anymore in LHCb. As HepMC2 and HepMC3 are incompatible and cannot be used in a project simultaneously they have to be removed to proceed with the porting of Gauss.

After a review to determine which parts are still required, they will need to be ported to HepMC3 and can then placed directly into future Gauss. Most prominently, this will include the LoKi functors for HepMC used for generator level cuts.

Marked this as WIP for now as it probably breaks other projects. @cattanem Is there a slot we can test this in?