Drop the comment

Author: maropu

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/UpdateAttributeNullability.scala

@@ -29,9 +29,6 @@ import org.apache.spark.sql.catalyst.rules.Rule
  * nullable field can be actually set as non-nullable, which cause illegal optimization
  * (e.g., NULL propagation) and wrong answers.
  * See SPARK-13484 and SPARK-13801 for the concrete queries of this case.
- *
- * This rule should be executed again at the end of optimization phase, as optimizer may change
- * some expressions and their nullabilities as well. See SPARK-21351 for more details.
  */
 object UpdateAttributeNullability extends Rule[LogicalPlan] {