[SPARK-23259][SQL] Clean up legacy code around hive external catalog …

…and HiveClientImpl

## What changes were proposed in this pull request?

Three legacy statements are removed by this patch:

- in HiveExternalCatalog: The withClient wrapper is not necessary for the private method getRawTable.

- in HiveClientImpl: There are some redundant code in both the tableExists and getTableOption method.

This PR takes over #20425

## How was this patch tested?

Existing tests

Closes #20425

Author: hyukjinkwon <gurwls223@apache.org>

Closes #21780 from HyukjinKwon/SPARK-23259.

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

@@ -114,7 +114,7 @@ private[spark] class HiveExternalCatalog(conf: SparkConf, hadoopConf: Configurat
    * should interpret these special data source properties and restore the original table metadata
    * before returning it.
    */
-  private[hive] def getRawTable(db: String, table: String): CatalogTable = withClient {
+  private[hive] def getRawTable(db: String, table: String): CatalogTable = {
     client.getTable(db, table)
   }
 

sql/hive/src/main/scala/org/apache/spark/sql/hive/client/HiveClientImpl.scala

@@ -353,15 +353,19 @@ private[hive] class HiveClientImpl(
     client.getDatabasesByPattern(pattern).asScala
   }
 
+  private def getRawTableOption(dbName: String, tableName: String): Option[HiveTable] = {
+    Option(client.getTable(dbName, tableName, false /* do not throw exception */))
+  }
+
   override def tableExists(dbName: String, tableName: String): Boolean = withHiveState {
-    Option(client.getTable(dbName, tableName, false /* do not throw exception */)).nonEmpty
+    getRawTableOption(dbName, tableName).nonEmpty
   }
 
   override def getTableOption(
       dbName: String,
       tableName: String): Option[CatalogTable] = withHiveState {
     logDebug(s"Looking up $dbName.$tableName")
-    Option(client.getTable(dbName, tableName, false)).map { h =>
+    getRawTableOption(dbName, tableName).map { h =>
       // Note: Hive separates partition columns and the schema, but for us the
       // partition columns are part of the schema
       val cols = h.getCols.asScala.map(fromHiveColumn)