Use Iterator from the prelude

[braddunbar]

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

error: the item `Iterator` is imported redundantly
  --> src/bytes_mut.rs:1:32
   |
1  | use core::iter::{FromIterator, Iterator};
   |                                ^^^^^^^^
   |
  ::: /home/runner/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/prelude/mod.rs:28:13
   |
28 |     pub use super::v1::*;
   |             --------- the item `Iterator` is already defined here
   |
   = note: `-D unused-imports` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(unused_imports)]`

[taiki-e]

Thanks!