Remove unused import

Author: sryza

core/src/main/scala/org/apache/spark/util/SizeEstimator.scala

@@ -25,7 +25,6 @@ import scala.collection.mutable.ArrayBuffer
 import scala.runtime.ScalaRunTime
 
 import org.apache.spark.Logging
-import org.apache.spark.annotation.DeveloperApi
 import org.apache.spark.util.collection.OpenHashSet