[SPARK-6103][Graphx]remove unused class to import in EdgeRDDImpl

Class TaskContext is unused in EdgeRDDImpl, so we need to remove it from import list.

Author: Lianhui Wang <lianhuiwang09@gmail.com>

Closes apache#4846 from lianhuiwang/SPARK-6103 and squashes the following commits:

31aed64 [Lianhui Wang] remove unused class to import in EdgeRDDImpl

graphx/src/main/scala/org/apache/spark/graphx/impl/EdgeRDDImpl.scala

@@ -19,7 +19,7 @@ package org.apache.spark.graphx.impl
 
 import scala.reflect.{classTag, ClassTag}
 
-import org.apache.spark.{OneToOneDependency, HashPartitioner, TaskContext}
+import org.apache.spark.{OneToOneDependency, HashPartitioner}
 import org.apache.spark.rdd.RDD
 import org.apache.spark.storage.StorageLevel