Simplify Jamba state management

[tlrmchlsmth]

Simplify Jamba state management by getting rid of the bookkeeping introduced in #6739. Once we do something like this, I think it will be easier to incorporate the Mamba cache into a more vLLM-native block manager state management.

Depends on #8012 and #8039

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[tlrmchlsmth]

Superseded by #9189