[SPARK-27479][BUILD] Hide API docs for org.apache.spark.util.kvstore

## What changes were proposed in this pull request?

The API docs should not include the "org.apache.spark.util.kvstore" package because they are internal private APIs. See the doc link: https://spark.apache.org/docs/latest/api/java/org/apache/spark/util/kvstore/LevelDB.html

## How was this patch tested?
N/A

Closes apache#24386 from gatorsmile/rmDoc.

Authored-by: gatorsmile <[email protected]>
Signed-off-by: gatorsmile <[email protected]>
(cherry picked from commit 61feb16)
Signed-off-by: gatorsmile <[email protected]>

Author: gatorsmile

project/SparkBuild.scala

@@ -676,6 +676,7 @@ object Unidoc {
       .map(_.filterNot(_.getCanonicalPath.contains("org/apache/spark/unsafe")))
       .map(_.filterNot(_.getCanonicalPath.contains("python")))
       .map(_.filterNot(_.getCanonicalPath.contains("org/apache/spark/util/collection")))
+      .map(_.filterNot(_.getCanonicalPath.contains("org/apache/spark/util/kvstore")))
       .map(_.filterNot(_.getCanonicalPath.contains("org/apache/spark/sql/catalyst")))
       .map(_.filterNot(_.getCanonicalPath.contains("org/apache/spark/sql/execution")))
       .map(_.filterNot(_.getCanonicalPath.contains("org/apache/spark/sql/internal")))