[SPARK-22720][SS] Make EventTimeWatermark Extend UnaryNode

## What changes were proposed in this pull request?
Our Analyzer and Optimizer have multiple rules for `UnaryNode`. After making `EventTimeWatermark` extend `UnaryNode`, we do not need a special handling for `EventTimeWatermark`.

## How was this patch tested?
The existing tests

Author: gatorsmile <[email protected]>

Closes #19913 from gatorsmile/eventtimewatermark.

Author: gatorsmile

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/plans/logical/EventTimeWatermark.scala

@@ -38,7 +38,7 @@ object EventTimeWatermark {
 case class EventTimeWatermark(
     eventTime: Attribute,
     delay: CalendarInterval,
-    child: LogicalPlan) extends LogicalPlan {
+    child: LogicalPlan) extends UnaryNode {
 
   // Update the metadata on the eventTime column to include the desired delay.
   override val output: Seq[Attribute] = child.output.map { a =>
@@ -60,6 +60,4 @@ case class EventTimeWatermark(
       a
     }
   }
-
-  override val children: Seq[LogicalPlan] = child :: Nil
 }