set chunked_prefill off when use mla

[DragonFive]

FIX #13370 (link existing issues this PR will resolve)

in vllm/config.py , it will forcing chunked prefill and prefix caching to be disabled, but it's too late, the max_num_batched_tokens will be set 2048 by default when user use --enable-chunked-prefill for mla attention model

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[tlrmchlsmth]

Thanks for the contribution!

Chunked prefill support for MLA #12639 will likely land today, so I don't think we need to do this. Otherwise I think this would be a good move. (Not closing this PR though, in case some last minute issue pops up with #12639)