Remove dead code

[taiki-e]

error: method `with` is never used
  --> src/sync.rs:65:12
   |
62 |     pub(crate) trait UnsafeCellExt {
   |                      ------------- method in this trait
...
65 |         fn with<R, F>(&self, f: F) -> R
   |            ^^^^
   |
   = note: `-D dead-code` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(dead_code)]`