[SPARK-21699][SQL] Remove unused getTableOption in ExternalCatalog

## What changes were proposed in this pull request?
This patch removes the unused SessionCatalog.getTableMetadataOption and ExternalCatalog. getTableOption.

## How was this patch tested?
Removed the test case.

Author: Reynold Xin <rxin@databricks.com>

Closes apache#18912 from rxin/remove-getTableOption.

Author: rxin

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/catalog/ExternalCatalog.scala

@@ -167,8 +167,6 @@ abstract class ExternalCatalog
 
   def getTable(db: String, table: String): CatalogTable
 
-  def getTableOption(db: String, table: String): Option[CatalogTable]
-
   def tableExists(db: String, table: String): Boolean
 
   def listTables(db: String): Seq[String]

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/catalog/InMemoryCatalog.scala

@@ -326,10 +326,6 @@ class InMemoryCatalog(
     catalog(db).tables(table).table
   }
 
-  override def getTableOption(db: String, table: String): Option[CatalogTable] = synchronized {
-    if (!tableExists(db, table)) None else Option(catalog(db).tables(table).table)
-  }
-
   override def tableExists(db: String, table: String): Boolean = synchronized {
     requireDbExists(db)
     catalog(db).tables.contains(table)

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/catalog/SessionCatalog.scala

@@ -387,9 +387,10 @@ class SessionCatalog(
 
   /**
    * Retrieve the metadata of an existing permanent table/view. If no database is specified,
-   * assume the table/view is in the current database. If the specified table/view is not found
-   * in the database then a [[NoSuchTableException]] is thrown.
+   * assume the table/view is in the current database.
    */
+  @throws[NoSuchDatabaseException]
+  @throws[NoSuchTableException]
   def getTableMetadata(name: TableIdentifier): CatalogTable = {
     val db = formatDatabaseName(name.database.getOrElse(getCurrentDatabase))
     val table = formatTableName(name.table)
@@ -398,18 +399,6 @@ class SessionCatalog(
     externalCatalog.getTable(db, table)
   }
 
-  /**
-   * Retrieve the metadata of an existing metastore table.
-   * If no database is specified, assume the table is in the current database.
-   * If the specified table is not found in the database then return None if it doesn't exist.
-   */
-  def getTableMetadataOption(name: TableIdentifier): Option[CatalogTable] = {
-    val db = formatDatabaseName(name.database.getOrElse(getCurrentDatabase))
-    val table = formatTableName(name.table)
-    requireDbExists(db)
-    externalCatalog.getTableOption(db, table)
-  }
-
   /**
    * Load files stored in given path into an existing metastore table.
    * If no database is specified, assume the table is in the current database.

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/catalog/SessionCatalogSuite.scala

@@ -510,17 +510,6 @@ abstract class SessionCatalogSuite extends AnalysisTest {
     }
   }
 
-  test("get option of table metadata") {
-    withBasicCatalog { catalog =>
-      assert(catalog.getTableMetadataOption(TableIdentifier("tbl1", Some("db2")))
-        == Option(catalog.externalCatalog.getTable("db2", "tbl1")))
-      assert(catalog.getTableMetadataOption(TableIdentifier("unknown_table", Some("db2"))).isEmpty)
-      intercept[NoSuchDatabaseException] {
-        catalog.getTableMetadataOption(TableIdentifier("tbl1", Some("unknown_db")))
-      }
-    }
-  }
-
   test("lookup table relation") {
     withBasicCatalog { catalog =>
       val tempTable1 = Range(1, 10, 1, 10)

sql/hive/src/main/scala/org/apache/spark/sql/hive/HiveExternalCatalog.scala

@@ -669,10 +669,6 @@ private[spark] class HiveExternalCatalog(conf: SparkConf, hadoopConf: Configurat
     restoreTableMetadata(getRawTable(db, table))
   }
 
-  override def getTableOption(db: String, table: String): Option[CatalogTable] = withClient {
-    client.getTableOption(db, table).map(restoreTableMetadata)
-  }
-
   /**
    * Restores table metadata from the table properties. This method is kind of a opposite version
    * of [[createTable]].