[WIP][SPARK-35380][SQL] Loading SparkSessionExtensions from ServiceLoader

Author: yaooqinn

sql/core/src/main/scala/org/apache/spark/sql/SparkSession.scala

@@ -23,7 +23,6 @@ import java.util.concurrent.TimeUnit._
 import java.util.concurrent.atomic.{AtomicBoolean, AtomicReference}
 
 import scala.collection.JavaConverters._
-import scala.collection.mutable.ArrayBuffer
 import scala.reflect.runtime.universe.TypeTag
 import scala.util.control.NonFatal