Remove comment

Author: sjvanrossum

sdks/java/io/kafka/src/main/java/org/apache/beam/sdk/io/kafka/KafkaUnboundedReader.java

@@ -481,8 +481,6 @@ public TopicPartition topicPartition() {
     // Update consumedOffset and avgRecordSize
     void recordConsumed(long offset, int size) {
       nextOffset = offset + 1;
-
-      // This is always updated from single thread. Probably not worth making atomic.
       avgRecordSize.update(size);
     }