DelBiblio() currently does not retrieve future holds, and consequently does not call CancelReserve(). The consequences of this are minor, as it just means that old_reserves doesn't get a copy of deleted future holds (instead, the future holds are deleted via the FK constraint), but this could be a problem in the future if (say) CancelReserve() gets extended to initiate sending hold cancellation notices. An argument could be made that the default value of the all_dates parameter to GetReservesFromBiblionumber should be true, not false. If so, all_dates might be better named 'show_only_current_holds' or the like.
DelBiblio calls Koha->holds - this includes future holds