Inhibit unused_mut variables if unused_variables is also triggered

[djc]

Given the following code:

fn main() {
    let mut s = String::from("a");
}

(Playground.)

The current output is:

warning: unused variable: `s`
 [--> src/main.rs:2:13
](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021#)  |
2 |     let mut s = String::from("a");
  |             ^ help: if this is intentional, prefix it with an underscore: `_s`
  |
  = note: `#[warn(unused_variables)]` on by default

warning: variable does not need to be mutable
 [--> src/main.rs:2:9
](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021#)  |
2 |     let mut s = String::from("a");
  |         ----^
  |         |
  |         help: remove this `mut`
  |
  = note: `#[warn(unused_mut)]` on by default

warning: `playground` (bin "playground") generated 2 warnings

warning: unused variable: `s`
 [--> src/main.rs:2:13
](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021#)  |
2 |     let mut s = String::from("a");
  |             ^ help: if this is intentional, prefix it with an underscore: `_s`
  |
  = note: `#[warn(unused_variables)]` on by default

warning: variable does not need to be mutable

It's obvious that an unused variable does not need to be declared mutable, and the extra warning puts more cognitive burden on the reader to figure out what's relevant.

Found this on 1.60.0, but also still happens on nightly.