Delete Levanter Ray TPU infra (#5031)

## Summary
Pure deletion, part of the Ray removal tracked in #4453:
- `lib/levanter/src/levanter/infra/ray_tpu.py` (177 LOC) —
`@ray.remote(max_calls=1)` wrapper around `run_on_pod`
- `lib/levanter/infra/launch_on_ray.py` (190 LOC) — Levanter's Ray
launcher (`JobSubmissionClient`, `JobStatus`, `RunDockerOnPodConfig`)

Net: **−367 LOC**. The fray-driven TPU orchestration in
`lib/fray/src/fray/cluster/` replaces both for any remaining callers.

## Out of scope (follow-up)
`lib/levanter/docs/Getting-Started-TPU-VM.md` still references
`launch_on_ray` in 5 places. Left untouched here; tracked in #5029
alongside the other Ray-doc cleanups.

## Test plan
- [x] `./infra/pre-commit.py --all-files --fix` clean
- [x] `uvx pyrefly check` — 164 errors, same as pre-edit baseline on
main
- [x] `rg
'ray_tpu|launch_on_ray|run_docker_on_pod|RunDockerOnPodConfig|submit_tpu_job_on_ray'
lib/levanter/tests tests/` → zero hits (no targeted tests to run);
neighbor-import sanity check `import
levanter.infra.{docker,cli_helpers,tpus}` passes
- [x] `rg 'levanter\.infra\.ray_tpu|launch_on_ray' lib/ tests/
experiments/ scripts/ -g '*.py'` → zero Python hits
- [x] `rg '^\s*import ray\b|^\s*from ray\b' lib/levanter/` → zero hits

Related: #4453, #4953, #5029

Co-authored-by: Romain Yon <[email protected]>

Author: yonromai