[SPARK-11506][MLLIB] Removed redundant operation in Online LDA implementation

In file LDAOptimizer.scala:

line 441: since "idx" was never used, replaced unrequired zipWithIndex.foreach with foreach.

-      nonEmptyDocs.zipWithIndex.foreach { case ((_, termCounts: Vector), idx: Int) =>
+      nonEmptyDocs.foreach { case (_, termCounts: Vector) =>

Author: a1singh <a1singh@ucsd.edu>

Closes #9456 from a1singh/master.

mllib/src/main/scala/org/apache/spark/mllib/clustering/LDAOptimizer.scala

@@ -438,7 +438,7 @@ final class OnlineLDAOptimizer extends LDAOptimizer {
 
       val stat = BDM.zeros[Double](k, vocabSize)
       var gammaPart = List[BDV[Double]]()
-      nonEmptyDocs.zipWithIndex.foreach { case ((_, termCounts: Vector), idx: Int) =>
+      nonEmptyDocs.foreach { case (_, termCounts: Vector) =>
         val ids: List[Int] = termCounts match {
           case v: DenseVector => (0 until v.size).toList
           case v: SparseVector => v.indices.toList