Don't lint unused params for trivial RHS

This doesn't warn:
```
def f(x: Int) = ???
```
Other expressive RHS:
```
def f(x: Int) = ()
def f(x: Int) = throw new RuntimeException
def f(x: Int) = 42
def f(x: Int): Option[Int] = None
```
Narrowly-occupied types indicate an intention to discard inputs. Probably overrides are invited.