### What it does
Checks methods that contain a `self` argument but don't use it

### Why is this bad?
It may be clearer to define the method as an associated function instead
of an instance method if it doesn't require `self`.

### Example
```
struct A;
impl A {
    fn method(&self) {}
}
```

Could be written:

```
struct A;
impl A {
    fn method() {}
}
```