[SPARK-20405][SQL] Dataset.withNewExecutionId should be private

## What changes were proposed in this pull request?
Dataset.withNewExecutionId is only used in Dataset itself and should be private.

## How was this patch tested?
N/A - this is a simple visibility change.

Author: Reynold Xin <rxin@databricks.com>

Closes #17699 from rxin/SPARK-20405.

sql/core/src/main/scala/org/apache/spark/sql/Dataset.scala

@@ -2778,7 +2778,7 @@ class Dataset[T] private[sql](
    * Wrap a Dataset action to track all Spark jobs in the body so that we can connect them with
    * an execution.
    */
-  private[sql] def withNewExecutionId[U](body: => U): U = {
+  private def withNewExecutionId[U](body: => U): U = {
     SQLExecution.withNewExecutionId(sparkSession, queryExecution)(body)
   }