Revert "fix: rayon imports"

This reverts commit 13e0333.

halo2_proofs/src/poly.rs

@@ -8,7 +8,7 @@ use crate::plonk::Assigned;
 use crate::SerdeFormat;
 use group::ff::{BatchInvert, Field};
 #[cfg(feature = "parallel-poly-read")]
-use maybe_rayon::{iter::ParallelIterator, prelude::ParallelSlice};
+use maybe_rayon::{iter::ParallelIterator, prelude::ParallelSliceMut};
 use std::fmt::Debug;
 use std::io;
 use std::marker::PhantomData;