Update ApplicationMaster.scala

Author: AngersZhuuuu

resource-managers/yarn/src/main/scala/org/apache/spark/deploy/yarn/ApplicationMaster.scala

@@ -35,7 +35,6 @@ import org.apache.hadoop.util.StringUtils
 import org.apache.hadoop.yarn.api._
 import org.apache.hadoop.yarn.api.records._
 import org.apache.hadoop.yarn.conf.YarnConfiguration
-import org.apache.hadoop.yarn.exceptions.ApplicationAttemptNotFoundException
 import org.apache.hadoop.yarn.util.{ConverterUtils, Records}
 
 import org.apache.spark._