`Zlhtrap` should be commented out (ifdefed) from the current deliverable

[lfiolhais]

The topic of horizontal traps is much broader than ACE and is in active
discussion in different TGs. I'd comment it out from the current specification
such that the freezes focuses on the ACE core primitives.

[radimkrcmar]

Horizontal traps are very easily implemented by higher privilege levels. The ISA just has to trap and the rest will be handled in software (common operating systems have mature horizontal trap support).
Hardware-accelerated horizontal traps would be purely for performance, and the section makes it look that performance of the trap is not really an issue, so I agree that we can ignore the horizontal trap acceleration.