Fix #717: deprecate unary_! on arbitrary js.Any.

It is redefined as not deprecated on js.Dynamic and js.prim.Boolean.
Maybe: "Considered abuse in typed JavaScript, will be removed ... ". I think what we want to say is "If the result of this is useful in any way to you, you are working in an untyped manner (and henceforth should use `js.Dynamic`) or with booleans (and henceforth should use `js.prim.Boolean` or `scala.Boolean`)", which is indeed hard to convey in a short message.