Clippy warning about unused lifetime should be fixed

[Darksonn]

A new version of clippy reports the following warning:

warning: this lifetime isn't used in the impl
   --> tokio/src/sync/semaphore.rs:634:6
    |
634 | impl<'a> Drop for SemaphorePermit<'_> {
    |      ^^
    |
    = note: `#[warn(clippy::extra_unused_lifetimes)]` on by default
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#extra_unused_lifetimes

We are currently using an older clippy so we don't get the warning on CI, but we cannot upgrade clippy until this warning is fixed.