SPARK-2519 part 2. Remove pattern matching on Tuple2 in critical section...

...s of CoGroupedRDD and PairRDDFunctions

This also removes an unnecessary tuple creation in cogroup.
And yeah as Reynold pointed out, this won't actually cast. A Seq is not a tuple4, you need to make a new one. In this case it might be okay to leave the code as is, because the non-pattern-matching way will be hairier. Basically you'd have to do `seq => (seq(0).asInstanceOf[Seq[V]], seq(1).asInstanceOf[Seq[W1]], etc)`.