[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

core/src/main/scala/org/apache/spark/deploy/SparkSubmit.scala

@@ -261,7 +261,7 @@ object SparkSubmit {
     }
 
     // In yarn-cluster mode, use yarn.Client as a wrapper around the user class
-    if (clusterManager == YARN && deployMode == CLUSTER) {
+    if (isYarnCluster) {
       childMainClass = "org.apache.spark.deploy.yarn.Client"
       if (args.primaryResource != SPARK_INTERNAL) {
         childArgs += ("--jar", args.primaryResource)