chore: fix nightly clippy rule needless_lifetimes

[yukiiiteru]

Motivation

The latest cargo-clippy adds a new rule:

error: the following explicit lifetimes could be elided: 's, 'cx
  --> volo/src/loadbalance/layer.rs:60:19
   |
60 |     async fn call<'s, 'cx>(
   |                   ^^  ^^^
61 |         &'s self,
   |          ^^
62 |         cx: &'cx mut Cx,
   |              ^^^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_lifetimes
   = note: `-D clippy::needless-lifetimes` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(clippy::needless_lifetimes)]`

Solution

Fix them