Draft: ATR-23431: Remove propagation of incorrect combo hypo

The code was previously propagating any combo hypo that already existed on one leg for combined chains (though even this was incorrect because only the ComboHypo for the last leg was propagated). This led to a Passthrough ComboHypo being propagated to the full combined step for X+jet chains.

The propagation has been removed and proper decisions are restored.

One possible catch: what to do if we have X + Y + Z, where X+Y are pre-merged and have a ComboHypo? e.g. tau1_tau2_dRtautau_jet. I don't think we have any working cases like this, yet, so nothing will break - just something to think about in the future.

Will remove draft when I push new references.

edit: this doesn't work, because this merging is also used by X + Y chains with a dedicated combo hypo, e.g. bphys chains... TBC.