[spec] remove unused args in spec decoding

[AlpinDale]

No description provided.

[gemini-code-assist]

Code Review

This pull request focuses on improving code quality by removing unused arguments from the speculative decoding configuration. Specifically, disable_logprobs, enable_chunked_prefill, and disable_log_stats have been removed from SpeculativeConfig as they were not being used, which cleans up the configuration and reduces potential confusion. The changes are consistently applied across configuration and argument parsing files. Additionally, a helpful assertion was added in the OpenAI API server to ensure an engine client instance is valid before use, enhancing the code's robustness. Overall, these are good maintainability improvements and I see no issues.