[SPARK-43118][SS] Remove unnecessary assert for UninterruptibleThread in KafkaMicroBatchStream

[jerrypeng]

What changes were proposed in this pull request?

The assert

assert(Thread.currentThread().isInstanceOf[UninterruptibleThread])

found https://github.com/apache/spark/blob/master/connector/kafka-0-10-sql/src/main/scala/org/apache/spark/sql/kafka010/KafkaMicroBatchStream.scala#L239

is not needed. The reason is the following

1. This assert was put there due to some issues when the old and deprecated KafkaOffsetReaderConsumer is used. The default offset reader implementation has been changed to KafkaOffsetReaderAdmin which no longer require it run via UninterruptedThread.
2. Even if the deprecated KafkaOffsetReaderConsumer is used, there are already asserts in that impl to check if it is running via UninterruptedThread e.g. https://github.com/apache/spark/blob/master/connector/kafka-0-10-sql/src/main/scala/org/apache/spark/sql/kafka010/KafkaOffsetReaderConsumer.scala#L130 thus the assert in KafkaMicroBatchStream is redundant.

Why are the changes needed?

Remove unnecessary assert. Clean up code.

Does this PR introduce any user-facing change?

No

How was this patch tested?

n/a

[HeartSaVioR]

+1 pending CI.

[HeartSaVioR]

Thanks! Merging to master.