Delete unused TASK_ID_MAPPING support (#8604)
This is currently unused. It existed for persistent caching, but it
sounds like we likely won't go with this mapping approach:
> It was used for persistent caching, but it looks like we probably take
a different approach (same ids between persistent cache and memory
cache). So you can remove it.
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https://vercel.slack.com/archives/C03EWR7LGEN/p1719340628045969?thread_ts=1719338013.257509&cid=C03EWR7LGEN
