'Avoid lambda' suggests unnecessary parens

It seems a bug was introduced in #1634 .

This was tested on a build of hlint from the default branch, from just now.

Hlint output (should be 'no hints'):
```
A.hs:3:15-27: Suggestion: Avoid lambda
Found:
  \ _ -> pure ()
Perhaps:
  (\ _ -> pure ())

1 hint
```

Source:
```
{-# LANGUAGE BlockArguments #-}
main :: IO ()
main = ($ 42) \_ -> pure ()
```