Use Iterator from the prelude (#673)

CI is [failing][failure] due to unused_imports because Iterator is
already in the prelude. Removing it fixes things up.

[failure]: https://github.com/tokio-rs/bytes/actions/runs/8034858583/job/21946873895

src/bytes_mut.rs

@@ -1,4 +1,4 @@
-use core::iter::{FromIterator, Iterator};
+use core::iter::FromIterator;
 use core::mem::{self, ManuallyDrop, MaybeUninit};
 use core::ops::{Deref, DerefMut};
 use core::ptr::{self, NonNull};