Suppress unused do-binding if discarded variable is Any or ZonkAny.

Consider example (#25895):

>  do { forever (return ()); blah }

where `forever :: forall a b. IO a -> IO b`.

Nothing constrains `b`, so it will be instantiates with `Any` or
`ZonkAny`.
But we certainly don't want to complain about a discarded do-binding.

Fixes #25895