Delete dead Marin Ray glue (#4453)

* Remove lib/marin/src/marin/cluster/ray.py (959 LOC, zero callers)
* Strip two lazy `import ray` branches in lib/marin/src/marin/execution/executor.py; the RemoteCallable (fray v2) dispatch path replaces them

Note: lib/marin/src/marin/run/ray_run.py is intentionally NOT deleted
here — it is still the documented launcher for ferries, tootsie runs,
and the add_scaling_heuristic recipe. It will be removed once the
production Ray cluster retires.

Author: yonromai