Rspack: Remove bincode dependency from rspack by way of turbo-unix-path (#98855)
https://github.com/vercel/next.js/pull/98440 unintentionally caused the
rspack `Cargo.lock` to become outdated because it added a `smallvec`
dependency to `turbo-unix-path`, which gets used by `next-taskless`.
We only really need `smallvec`'s `serde` and `bincode_impl` features
when used with turbo-tasks, so move the feature declaration there.