[SPARK-3589][Minor]remove redundant code

https://issues.apache.org/jira/browse/SPARK-3589

"export CLASSPATH" in spark-class is redundant since same variable is exported before.
We could reuse defined value "isYarnCluster" in SparkSubmit.scala.
Oh I see, we already exported it out there. I put this to make it more explicit that these are the variables needed by `SparkSubmitDriverBootstrapper`, so even though this line is somewhat redundant, it's here for code readability reasons.