Drop triple X for cpu affinity for OpenBSD

Drop triple X for cpu affinity for OpenBSD

(Sorry, resending this as gmail refused an email title including triple X)

This isn't really a fio's TODO item since no such thing seems to
exist (or have existed) in OpenBSD as of 2017, according to below
reference.

NetBSD is in more or less similar situation, except that NetBSD
has pthread_[gs]etaffinity_np() for threads, though it has another
problem that they don't take pid/tid.

CPU affinity comes to OpenBSD (2009)
http://undeadly.org/cgi?action=article&sid=20090324210236

tedu old comment about cpu affinity. (2012)
http://openbsd-archive.7691.n7.nabble.com/tedu-old-comment-about-cpu-affinity-tt172330.html

how to use cpu affinity from user space (2013)
http://openbsd-archive.7691.n7.nabble.com/how-to-use-cpu-affinity-from-user-space-td222323.html

Signed-off-by: Tomohiro Kusumi <tkusumi@tuxera.com>
Signed-off-by: Jens Axboe <axboe@fb.com>