[SPARK-17346][SQL][TESTS] Fix the flaky topic deletion in KafkaSourceStressSuite

## What changes were proposed in this pull request?

A follow up Pr for SPARK-17346 to fix flaky `org.apache.spark.sql.kafka010.KafkaSourceStressSuite`.

Test log: https://amplab.cs.berkeley.edu/jenkins/job/spark-master-test-sbt-hadoop-2.4/1855/testReport/junit/org.apache.spark.sql.kafka010/KafkaSourceStressSuite/_It_is_not_a_test_/

Looks like deleting the Kafka internal topic `__consumer_offsets` is flaky. This PR just simply ignores internal topics.

## How was this patch tested?

Existing tests.

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #15384 from zsxwing/SPARK-17346-flaky-test.