SPARK-1160: Deprecate toArray in RDD

https://spark-project.atlassian.net/browse/SPARK-1160

reported by @mateiz: "It's redundant with collect() and the name doesn't make sense in Java, where we return a List (we can't return an array due to the way Java generics work). It's also missing in Python."

In this patch, I deprecated the method and changed the source files using it by replacing toArray with collect() directly