Adjust comment atop ExecShutdownNode.

After commits a315b967 and b805b63ac2, part of the comment atop
ExecShutdownNode is redundant.  Adjust it.

Author: Amit Kapila
Backpatch-through: 10 where both the mentioned commits are present.
Discussion: https://postgr.es/m/86137f17-1dfb-42f9-7421-82fd786b04a1@anayrat.info

src/backend/executor/execProcnode.c

@@ -736,11 +736,7 @@ ExecEndNode(PlanState *node)
  * ExecShutdownNode
  *
  * Give execution nodes a chance to stop asynchronous resource consumption
- * and release any resources still held.  Currently, this is only used for
- * parallel query, but we might want to extend it to other cases also (e.g.
- * FDW).  We might also want to call it sooner, as soon as it's evident that
- * no more rows will be needed (e.g. when a Limit is filled) rather than only
- * at the end of ExecutorRun.
+ * and release any resources still held.
  */
 bool
 ExecShutdownNode(PlanState *node)