[SPARK-3279] Remove useless field variable in ApplicationMaster

Author: Kousuke Saruta <sarutak@oss.nttdata.co.jp>

Closes #2177 from sarutak/SPARK-3279 and squashes the following commits:

2955edc [Kousuke Saruta] Removed useless field variable from ApplicationMaster

yarn/common/src/main/scala/org/apache/spark/deploy/yarn/ApplicationMaster.scala

@@ -46,7 +46,6 @@ private[spark] class ApplicationMaster(args: ApplicationMasterArguments,
   client: YarnRMClient) extends Logging {
   // TODO: Currently, task to container is computed once (TaskSetManager) - which need not be
   // optimal as more containers are available. Might need to handle this better.
-  private val ALLOCATE_HEARTBEAT_INTERVAL = 100
 
   private val sparkConf = new SparkConf()
   private val yarnConf: YarnConfiguration = new YarnConfiguration(new Configuration())