Remove the bogus _PREDICATE sort

[ttuegel]

Follow up on #1442.

We can remove this after we implement a specialized simplifier for Predicate.

[ttuegel]

We can remove this after we implement a specialized simplifier for Predicate.

I don't think this is really true, because the simplifier doesn't really look at the sort.

[ttuegel]

This task includes writing a new Predicate simplifier.