Remove Run 2 Flavour Tagging code and optimise GetFlavourTaggingParticles loop

Based comments in in !3160 (merged) between @sesen and @dtou, @dtou will remove the Run 2 flavour tagging code after the next release of Rec, presumably v35r6. @dtou will also implement @graven's comment in !3160 (merged) to optimise the GetFlavourTaggingParticles loop in the same MR that removes Run 2 flavour tagging code.