[MINOR] Follow-up to #12625

## What changes were proposed in this pull request?

That patch mistakenly widened the visibility from `private[x]` to `protected[x]`. This patch reverts those changes.

Author: Andrew Or <andrew@databricks.com>

Closes #12686 from andrewor14/visibility.

mllib/src/main/scala/org/apache/spark/ml/clustering/LDA.scala

@@ -360,10 +360,10 @@ private[clustering] trait LDAParams extends Params with HasFeaturesCol with HasM
  */
 @Since("1.6.0")
 @Experimental
-sealed abstract class LDAModel protected[ml] (
+sealed abstract class LDAModel private[ml] (
     @Since("1.6.0") override val uid: String,
     @Since("1.6.0") val vocabSize: Int,
-    @Since("1.6.0") @transient protected[ml] val sparkSession: SparkSession)
+    @Since("1.6.0") @transient private[ml] val sparkSession: SparkSession)
   extends Model[LDAModel] with LDAParams with Logging with MLWritable {
 
   // NOTE to developers:
@@ -512,7 +512,7 @@ sealed abstract class LDAModel protected[ml] (
  */
 @Since("1.6.0")
 @Experimental
-class LocalLDAModel protected[ml] (
+class LocalLDAModel private[ml] (
     uid: String,
     vocabSize: Int,
     @Since("1.6.0") override protected val oldLocalModel: OldLocalLDAModel,
@@ -604,7 +604,7 @@ object LocalLDAModel extends MLReadable[LocalLDAModel] {
  */
 @Since("1.6.0")
 @Experimental
-class DistributedLDAModel protected[ml] (
+class DistributedLDAModel private[ml] (
     uid: String,
     vocabSize: Int,
     private val oldDistributedModel: OldDistributedLDAModel,

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

@@ -41,7 +41,7 @@ import org.apache.spark.sql.types.StructType
  * @since 1.4.0
  */
 @Experimental
-class DataFrameReader protected[sql](sparkSession: SparkSession) extends Logging {
+class DataFrameReader private[sql](sparkSession: SparkSession) extends Logging {
 
   /**
    * Specifies the input data source format.

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

@@ -151,7 +151,7 @@ private[sql] object Dataset {
  *
  * @since 1.6.0
  */
-class Dataset[T] protected[sql](
+class Dataset[T] private[sql](
     @transient val sparkSession: SparkSession,
     @DeveloperApi @transient val queryExecution: QueryExecution,
     encoder: Encoder[T])