Remove redundant imports

[taiki-e]

This fixes CI failure on the latest nightly: https://github.com/rust-lang/portable-simd/actions/runs/8001283064/job/21852194555?pr=396

error: the item `TryFrom` is imported redundantly
 --> crates/core_simd/src/vector.rs:6:21
  |
6 | use core::convert::{TryFrom, TryInto};
  |                     ^^^^^^^
 --> /rustc/3406ada96f8e16e49e947a91db3eba0db45245fa/library/core/src/prelude/mod.rs:46:[30](https://github.com/rust-lang/portable-simd/actions/runs/8001283064/job/21852194555?pr=396#step:5:31)
  |
  = note: the item `TryFrom` is already defined here
  |
  = note: `-D unused-imports` implied by `-D warnings`
  = help: to override `-D warnings` add `#[allow(unused_imports)]`

error: the item `TryInto` is imported redundantly
 --> crates/core_simd/src/vector.rs:6:30
  |
6 | use core::convert::{TryFrom, TryInto};
  |                              ^^^^^^^
 --> /rustc/[34](https://github.com/rust-lang/portable-simd/actions/runs/8001283064/job/21852194555?pr=396#step:5:35)06ada96f8e16e49e947a91db3eba0db45245fa/library/core/src/prelude/mod.rs:46:39
  |
  = note: the item `TryInto` is already defined here