Removes TODO comments that are no longer applicable

- we do not want to use mutable updates because that would mean making vals
vars, losing various guarantees for final fields of the java memory model

- we do not want to add more subcasses of HashSet, because that would make
callsites megamorphic. If anything, we should try to reduce the number of
subclasses

- checking for monomorphic call site is not worth it for a single virtual
method call.

Review by @Ichoran