FStar.Witnessed.Core: mark witnessed argument as @@@unused

[mtzguido]

This allows to user a witnessed token in recursive definitions in non-strictly positive positions. The justification is 1) that this token is really just a unit, it's the axiom that gives it any interesting behavior, and 2) that this token cannot be used except through an effectful operation.

So it should be safe to mark it its type argument @@@unused.