adk-2.0 producer: delete deprecated on_state_change_callback stub

Parent tracker: #190 (v15 contract).
Wave: 2 — Producer cleanup.

## Contract

`on_state_change_callback` in `bigquery_agent_analytics_plugin.py:3125` is deprecated, logs a warning, and is never called by the framework. Delete the stub and the warning path.

## Acceptance

- [ ] `on_state_change_callback` and its warning code removed.
- [ ] No remaining references in the plugin or tests.
- [ ] Existing tests pass.

## References

- #190 (v15 producer §7).
- `src/google/adk/plugins/bigquery_agent_analytics_plugin.py:3125`.