MAJOR: config: disable support for nbproc and nbthread in parallel

MAJOR: config: disable support for nbproc and nbthread in parallel

When 1.8 was released, we wanted to support both nbthread and nbproc to
observe how things would go. Since then it appeared obvious that the two
are never used together because of the pain to configure affinity in this
case, and instead of bringing benefits, it brings the limitations of both
models, and causes multiple threads to compete for the same CPU. In
addition, it costs a lot to support both in parallel, so let's get rid
of this once for all.