Don't bind threads to specific CPUs.

Binding threads, especially now that ZFS is not populating all CPUs with
threads (default 75% fill) means that for multiple pools there will
always be some CPUs that are never available for work. Worst case
we are looking at CPU 0 being most contended.

There are other (less serious) side-effects but for now I'm
going to just disable this feature. My dual core system with two
pools will thank me later.