Turbopack: chore: Remove dead experimental.ppr struct field (#85792)
- `experimental.ppr` was replaced with `cacheComponents`.
- It's still in the zod and TS type schemas, I think for our error messages on the Next.js side, but Turbopack only needs to know about the fields that Turbopack cares about. Serde silently ignores unknown fields by default.
- Nothing was reading this field from this struct.
- This unit test coverage is very overkill.