False positive: UnusedVariable for methods that unconditionally throw

### Description of the problem / feature request:

*UnusedVariable* triggers for a method like this:

```java
private static boolean compare(int left, int right)
{
throw new AssertionError("this method should not be called");
}
```

We can rename the parameters to `unusedLeft` and `unusedRight`, which is often a good way to signal intent to readers of the code, but that is redundant in this case, since it's immediately obvious that this method will not use the parameters.

### What version of Error Prone are you using?

2.4.0