sobelow should not accept `# sobelow_skip` that are not needed

I notice in our code an instance of

```
  # sobelow_skip ["XSS.Raw"]
  def a_function(arg) do
    that_does_not_call_raw()
  end
```

I believe `sobelow` should raise an error on these. They do not reflect the code / current intention. Although unlikely, they could allow someone to add `raw` without it being super apparent in the diff of the resulting PR.

Thanks for `sobelow`