Remove unused import.

Author: kunalkhamar

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

@@ -17,8 +17,6 @@
 
 package org.apache.spark.sql
 
-import scala.collection.mutable.ListBuffer
-
 import org.apache.spark.annotation.{Experimental, InterfaceStability}
 import org.apache.spark.sql.catalyst.plans.logical.LogicalPlan
 import org.apache.spark.sql.catalyst.rules.Rule