Enable clippy::redundant_locals (#8750)

This PR enables the
[`clippy::redundant_locals`](https://rust-lang.github.io/rust-clippy/master/index.html#/redundant_locals)
rule and fixes the outstanding violations.

Release Notes:

- N/A

crates/rpc/src/conn.rs

@@ -84,7 +84,6 @@ impl Connection {
             });
 
             let rx = rx.then({
-                let killed = killed;
                 let executor = executor.clone();
                 move |msg| {
                     let killed = killed.clone();