Same particle warning in Run2OSVertexChargeTagger

The following warning appears after merging !4025 (merged) in the test_davinci_tupling_with_flavour_tagging test

Run2OSVertexChargeTagger_96fcfa3... WARNING distance(p,p): the same particle
Run2OSVertexChargeTagger_96fcfa3... WARNING distance(p,p): the same particle

The reason for the warnings should be understood, and addressed.

@cprouve @masmith

Thanks @jonrob for noticing.