Refactor: scheduler: drop overwrite from sorted_set_t

overwrite was redundantly in both sorted_set_t and unpack_data_t. Since it
doesn't vary by nvpair block, it makes more sense to drop it from sorted_set_t
in favor of passing the unpack_data_t with g_list_sort_with_data().

Separately, drop the overwrite argument from make_pairs(), because it was
unused.