[cleanup] Remove prefill logprobs recomputation path

[maocheng23]

Stacked on top of #1088.

Summary

Removes the unused prefill-logprob recomputation path from Miles.

This was split out from the Qwen3 MoE SP+PP true-on-policy support PR so the functional SP+PP support and the controversial cleanup can be reviewed independently.

Main Changes

• Remove the --recompute-logprobs-via-prefill CLI flag and validation.
• Remove the rollout hooks that called SGLang prefill scoring after generation.
• Delete the SGLang prefill scoring helper and its dedicated fast tests.
• Remove test assertions that only existed to verify the flag was absent from true-on-policy launch plans.
• Keep --sglang-enable-prefill-only-deterministic-inference available for any future model that genuinely needs prefill-only deterministic inference.

Validation

Local split checks:

• git diff --check for [feat] Add SP and PP support for qwen_moe true on policy #1088 after the split
• git diff --check for this stacked removal PR
• python -m py_compile over changed Python files in [feat] Add SP and PP support for qwen_moe true on policy #1088 after the split
• python -m py_compile over changed Python files in this stacked removal PR

The prior focused pytest run for the affected true-on-policy/script tests hung locally without producing failures, so I stopped it and did not use it as validation evidence.

[gemini-code-assist]

Code Review

This pull request removes the --recompute-logprobs-via-prefill command-line argument and all associated logic, including the deletion of the utility module miles/rollout/generate_utils/prefill_logprobs.py and its corresponding unit tests. References to this functionality have been cleaned up across argument validation, rollout engines, and configuration tests. There are no review comments, so I have no feedback to provide.

[Zhichenzzz]

LGTM! Thanks.