[SPARK-16964][SQL] Remove private[sql] and private[spark] from sql.execution package

## What changes were proposed in this pull request?
This package is meant to be internal, and as a result it does not make sense to mark things as private[sql] or private[spark]. It simply makes debugging harder when Spark developers need to inspect the plans at runtime.

This patch removes all private[sql] and private[spark] visibility modifiers in org.apache.spark.sql.execution.

## How was this patch tested?
N/A - just visibility changes.

Author: Reynold Xin <rxin@databricks.com>

Closes #14554 from rxin/remote-private.