CASSANDRA-3104, this was removed with the following reasoning:
compaction_throughput_mb_per_sec is a more effective throttle on compaction.
This turns out to be false in the majority of deployments. In many (if not most) situations, compaction is actually CPU bound, not IO bound, so multithreaded compaction is generally helpful, but the priority needs to be lowered in order to prevent it from stealing CPU used for reads/writes.
Compaction is always CPU bound on both real hardware (sw raid0 with two SATA disks) and on a rackspace cloud server (though my understanding is they are back by a raid10 array underneath) however I suspect even a single drive is fast enough to handle the ~20MB/s that compaction is currently performing when unthrottled.