[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.

Author: WangTaoTheTonic <barneystinson@aliyun.com>

Closes #2445 from WangTaoTheTonic/removeRedundant and squashes the following commits:

6fb6872 [WangTaoTheTonic] remove redundant code

(cherry picked from commit 471e6a3a47bd4b94878798f6f6fc93e2e672efff)
Signed-off-by: Patrick Wendell <pwendell@gmail.com>