Check for args like `fn foo(ref x: u8)`, where the `ref` is useless

`ref` on an entire argument (as opposed to a substructure of an argument) is effectively ignored by `rustc`, as you only receive a reference to the value already copied as a parameter.

```rust
fn bleh(ref mut x: u8) {
}
```
```
warning: `ref` directly on a function argument is ignored. Have you considered using a reference type instead?, #[warn(clippy_toplevel_ref_arg)] on by default
fn bleh(ref mut x: u8) {
        ^~~~~~~~~
```