[SPARK-16371][SQL] Two follow-up tasks

## What changes were proposed in this pull request?
This is a small follow-up for SPARK-16371:

1. Hide removeMetadata from public API.
2. Add JIRA ticket number to test case name.

## How was this patch tested?
Updated a test comment.

Author: Reynold Xin <rxin@databricks.com>

Closes #14074 from rxin/parquet-filter.

sql/catalyst/src/main/scala/org/apache/spark/sql/types/StructType.scala

@@ -413,10 +413,10 @@ object StructType extends AbstractDataType {
     StructType(fields.asScala)
   }
 
-  protected[sql] def fromAttributes(attributes: Seq[Attribute]): StructType =
+  private[sql] def fromAttributes(attributes: Seq[Attribute]): StructType =
     StructType(attributes.map(a => StructField(a.name, a.dataType, a.nullable, a.metadata)))
 
-  def removeMetadata(key: String, dt: DataType): DataType =
+  private[sql] def removeMetadata(key: String, dt: DataType): DataType =
     dt match {
       case StructType(fields) =>
         val newFields = fields.map { f =>

sql/core/src/test/scala/org/apache/spark/sql/execution/datasources/parquet/ParquetFilterSuite.scala

@@ -544,7 +544,7 @@ class ParquetFilterSuite extends QueryTest with ParquetTest with SharedSQLContex
     }
   }
 
-  test("Do not push down filters incorrectly when inner name and outer name are the same") {
+  test("SPARK-16371 Do not push down filters when inner name and outer name are the same") {
     withParquetDataFrame((1 to 4).map(i => Tuple1(Tuple1(i)))) { implicit df =>
       // Here the schema becomes as below:
       //