Remove all usage of the CHLO dialect (from StableHLO).

[ScottTodd]

Lots of this code is legacy and we should standardize on a small number of well supported dialects where possible. It looks like we were/are using this for different lowering paths of the "TopK" operation, but it is hard to tell with our current sparse test coverage of JAX, TensorFlow, and StableHLO if these lowering paths are still in use:

• https://github.com/iree-org/iree-test-suites/issues/3
• https://github.com/iree-org/iree-test-suites/issues/4
• JAX tests (no issue filed)

See also this discussion on Discord.