[SPARK-26271][FOLLOW-UP][SQL] remove unuse object SparkPlan

## What changes were proposed in this pull request?

this code come from PR: #11190,
but this code has never been used, only since  PR: #14548,
Let's continue fix it. thanks.

## How was this patch tested?

N / A

Closes #23227 from heary-cao/unuseSparkPlan.

Authored-by: caoxuewen <cao.xuewen@zte.com.cn>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>

sql/core/src/main/scala/org/apache/spark/sql/execution/SparkPlan.scala

@@ -423,11 +423,6 @@ abstract class SparkPlan extends QueryPlan[SparkPlan] with Logging with Serializ
   }
 }
 
-object SparkPlan {
-  private[execution] val subqueryExecutionContext = ExecutionContext.fromExecutorService(
-    ThreadUtils.newDaemonCachedThreadPool("subquery", 16))
-}
-
 trait LeafExecNode extends SparkPlan {
   override final def children: Seq[SparkPlan] = Nil
   override def producedAttributes: AttributeSet = outputSet