SPARK-2738. Remove redundant imports in BlockManagerSuite

Author: Sandy Ryza <sandy@cloudera.com>

Closes #1642 from sryza/sandy-spark-2738 and squashes the following commits:

a923e4e [Sandy Ryza] SPARK-2738. Remove redundant imports in BlockManagerSuite

core/src/test/scala/org/apache/spark/storage/BlockManagerSuite.scala

@@ -21,9 +21,6 @@ import java.nio.{ByteBuffer, MappedByteBuffer}
 import java.util.Arrays
 
 import akka.actor._
-import org.apache.spark.SparkConf
-import org.apache.spark.serializer.{JavaSerializer, KryoSerializer}
-import org.apache.spark.util.{AkkaUtils, ByteBufferInputStream, SizeEstimator, Utils}
 import org.mockito.Mockito.{mock, when}
 import org.scalatest.{BeforeAndAfter, FunSuite, PrivateMethodTester}
 import org.scalatest.concurrent.Eventually._