Disable predicate subtyping:

Disable predicate subtyping:
* remove from public interface (ExprManager, Type)
* CVC parser reports an unimplemented feature error if used

I didn't want to tear it out completely (from NodeManager, TypeNode,
type-checking, pre-processing, etc.) because that's a lot of hassle
and we'll add it back in after the release anyway.  It *does* mean
that CVC4::Predicate is in the public interface, but that it can't be
used for anything (by users).

(this commit was certified error- and warning-free by the test-and-commit script.)