Remove client stats from canonical form.

The type signature of `work` was <client_data, state> -> <client_updates, client_stats> where client_updates = <simple_agg, secure_agg>. After this CL, the type signature of `work` becomes <client_data, state> -> client_updates.

* `canonical_form.py`: remove `client_outputs` from `work`; update doc.

* `canonical_form_utils.py`: remove c8 from the canonical form, and change c7 to c6 and c6 to c5; remove `_create_next_with_fake_client_output` function.

* Remove `client_outputs` from `test_utils.py`.

PiperOrigin-RevId: 319873694