Removed unused return value in SparkContext.runJob

Return type of this `runJob` version is `Unit`:

    def runJob[T, U: ClassManifest](
        rdd: RDD[T],
        func: (TaskContext, Iterator[T]) => U,
        partitions: Seq[Int],
        allowLocal: Boolean,
        resultHandler: (Int, U) => Unit) {
        ...
    }

It's obviously unnecessary to "return" `result`.