Simplify xAOD true particle

Andrii Verbytskyirequested to merge
averbyts/athena:removetph into master

Simplify xAOD true particle

Tag @abuckley @christos @jchapman

The idea is to create a member function instead of multiple calls to MC::isGenStable. Therefore, the code that defines isGenStable is locatized in that function. This also removes the need of the header TruthParticleHelpers.h, which is not used anywhere else.

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