SI-7391 Always use ForkJoin in Scala actors on ...

... Java 6 and above (except when the porperty
actors.enableForkJoin says otherwise)

Like SI-7236 and SI-7237, the logic in
scala.actors.scheduler.ThreadPoolConfig.useForkJoin
(which resulted in a different thread pool implementation
being chosen) was causing random hangs in the test
concurrent-stream.scala when running on Avian.