[SPARK-24963][K8S][TESTS] Don't set service account name for client mode test

## What changes were proposed in this pull request?

Don't set service account name for the pod created in client mode

## How was this patch tested?

Test should continue running smoothly in Jenkins.

Author: mcheah <[email protected]>

Closes apache#21900 from mccheah/fix-integration-test-service-account.

Author: mccheah

resource-managers/kubernetes/integration-tests/src/test/scala/org/apache/spark/deploy/k8s/integrationtest/ClientModeTestsSuite.scala

@@ -61,7 +61,6 @@ trait ClientModeTestsSuite { k8sSuite: KubernetesSuite =>
           .withLabels(labels.asJava)
           .endMetadata()
         .withNewSpec()
-          .withServiceAccountName("default")
           .addNewContainer()
             .withName("spark-example")
             .withImage(image)