[SPARK-17089][DOCS] Remove api doc link for mapReduceTriplets operator

## What changes were proposed in this pull request?

Remove the api doc link for mapReduceTriplets operator because in latest api they are remove so when user link to that api they will not get mapReduceTriplets there so its more good to remove than confuse the user.

## How was this patch tested?
Run all the test cases

![screenshot from 2016-08-16 23-08-25](https://cloud.githubusercontent.com/assets/8075390/17709393/8cfbf75a-6406-11e6-98e6-38f7b319d833.png)

Author: sandy <[email protected]>

Closes apache#14669 from phalodi/SPARK-17089.

Author: phalodi

docs/graphx-programming-guide.md

@@ -24,7 +24,6 @@ description: GraphX graph processing library guide for Spark SPARK_VERSION_SHORT
 [Graph.outerJoinVertices]: api/scala/index.html#org.apache.spark.graphx.Graph@outerJoinVertices[U,VD2](RDD[(VertexId,U)])((VertexId,VD,Option[U])⇒VD2)(ClassTag[U],ClassTag[VD2]):Graph[VD2,ED]
 [Graph.aggregateMessages]: api/scala/index.html#org.apache.spark.graphx.Graph@aggregateMessages[A]((EdgeContext[VD,ED,A])⇒Unit,(A,A)⇒A,TripletFields)(ClassTag[A]):VertexRDD[A]
 [EdgeContext]: api/scala/index.html#org.apache.spark.graphx.EdgeContext
-[Graph.mapReduceTriplets]: api/scala/index.html#org.apache.spark.graphx.Graph@mapReduceTriplets[A](mapFunc:org.apache.spark.graphx.EdgeTriplet[VD,ED]=>Iterator[(org.apache.spark.graphx.VertexId,A)],reduceFunc:(A,A)=>A,activeSetOpt:Option[(org.apache.spark.graphx.VertexRDD[_],org.apache.spark.graphx.EdgeDirection)])(implicitevidence$10:scala.reflect.ClassTag[A]):org.apache.spark.graphx.VertexRDD[A]
 [GraphOps.collectNeighborIds]: api/scala/index.html#org.apache.spark.graphx.GraphOps@collectNeighborIds(EdgeDirection):VertexRDD[Array[VertexId]]
 [GraphOps.collectNeighbors]: api/scala/index.html#org.apache.spark.graphx.GraphOps@collectNeighbors(EdgeDirection):VertexRDD[Array[(VertexId,VD)]]
 [RDD Persistence]: programming-guide.html#rdd-persistence
@@ -596,7 +595,7 @@ compute the average age of the more senior followers of each user.
 ### Map Reduce Triplets Transition Guide (Legacy)
 
 In earlier versions of GraphX neighborhood aggregation was accomplished using the
-[`mapReduceTriplets`][Graph.mapReduceTriplets] operator:
+`mapReduceTriplets` operator:
 
 {% highlight scala %}
 class Graph[VD, ED] {
@@ -607,7 +606,7 @@ class Graph[VD, ED] {
 }
 {% endhighlight %}
 
-The [`mapReduceTriplets`][Graph.mapReduceTriplets] operator takes a user defined map function which
+The `mapReduceTriplets` operator takes a user defined map function which
 is applied to each triplet and can yield *messages* which are aggregated using the user defined
 `reduce` function.
 However, we found the user of the returned iterator to be expensive and it inhibited our ability to