Refactor: scheduler: drop redundant "top" XML element

expand_idref() works the same with its second argument NULL, since
get_xpath_object() always searches the entire XML document, and not just
beneath its "top" argument.

That means the populate_hash() and make_pairs() static functions don't need a
separate "top" argument, which then means unpack_data_t doesn't need a "top"
member.