Auto merge of #7225 - InquisitivePenguin:unnessecary-async, r=llogiq

Auto merge of #7225 - InquisitivePenguin:unnessecary-async, r=llogiq

New lint: `unused_async`

changelog: Adds a lint, `unused_async`, which checks for async functions with no await statements

`unused_async` is a lint that reduces code smell and overhead by encouraging async functions to be refactored into synchronous functions.

Fixes #7176

### Examples

```rust
async fn get_random_number() -> i64 {
    4 // Chosen by fair dice roll. Guaranteed to be random.
}
```

Could be written as:

```rust
fn get_random_number() -> i64 {
    4 // Chosen by fair dice roll. Guaranteed to be random.
}
```

Something like this, however, should **not** be caught by clippy:
```rust
#[async_trait]
trait AsyncTrait {
    async fn foo();
}

struct Bar;

#[async_trait]
impl AsyncTrait for Bar {
    async fn foo() {
        println!("bar");
    }
}
```