delete use std::iter::FromIterator to suppress warnings

[Neutron3529]

Currently, Rust automatically add std::iter::FromIterator since prelude of std always included if #![no_std] is not enabled.

Thus no need to add use std::iter::FromIterator

(actually, that line will trigger 2 warnings:)

warning: the item `FromIterator` is imported redundantly
   --> src/devel.rs:13:5
    |
13  | use std::iter::FromIterator;
    |     ^^^^^^^^^^^^^^^^^^^^^^^
    |
   ::: /home/neutron/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/prelude/mod.rs:129:13
    |
129 |     pub use core::prelude::rust_2021::*;
    |             ------------------------ the item `FromIterator` is already defined here
    |
    = note: `#[warn(unused_imports)]` on by default

warning: the item `FromIterator` is imported redundantly
   --> src/download.rs:8:5
    |
8   | use std::iter::FromIterator;
    |     ^^^^^^^^^^^^^^^^^^^^^^^
    |
   ::: /home/neutron/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/prelude/mod.rs:129:13
    |
129 |     pub use core::prelude::rust_2021::*;
    |             ------------------------ the item `FromIterator` is already defined here

warning: `paru` (lib) generated 2 warnings

Thus I create this PR to suppress them.