drop some redundant Ord method implementations
partial revert of 31fa865

Ratio is generic and so might contain a non-total orderable type. It
should not use the default Ord implementation.
auto merge of #8862 : dcrewi/rust/drop-redundant-ord-impls, r=thestinger

If this were to break something, I would expect existing tests to catch it, and a `make check` run passes locally.