[SPARK-6980] Cleaned up imports in AkkaRpcEnvSuite

Author: BryanCutler

core/src/test/scala/org/apache/spark/rpc/akka/AkkaRpcEnvSuite.scala

@@ -17,15 +17,8 @@
 
 package org.apache.spark.rpc.akka
 
-import scala.concurrent.Await
-import scala.concurrent.ExecutionContext.Implicits.global
-import scala.concurrent.duration._
 import scala.language.postfixOps
 
-import akka.actor.{Actor, ActorSystem, Props}
-import akka.pattern.ask
-import com.typesafe.config.{ConfigFactory, ConfigValueFactory}
-
 import org.apache.spark.{SecurityManager, SparkConf}
 import org.apache.spark.rpc._