[SPARK-16383][SQL] Remove SessionState.executeSql

## What changes were proposed in this pull request?

This PR removes `SessionState.executeSql` in favor of `SparkSession.sql`. We can remove this safely since the visibility `SessionState` is `private[sql]` and `executeSql` is only used in one **ignored** test, `test("Multiple Hive Instances")`.

## How was this patch tested?

Pass the Jenkins tests.

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes apache#14055 from dongjoon-hyun/SPARK-16383.

Author: dongjoon-hyun

sql/core/src/main/scala/org/apache/spark/sql/internal/SessionState.scala

@@ -24,7 +24,7 @@ import org.apache.hadoop.fs.Path
 
 import org.apache.spark.sql._
 import org.apache.spark.sql.catalyst.analysis.{Analyzer, FunctionRegistry}
-import org.apache.spark.sql.catalyst.catalog.{ArchiveResource, _}
+import org.apache.spark.sql.catalyst.catalog._
 import org.apache.spark.sql.catalyst.optimizer.Optimizer
 import org.apache.spark.sql.catalyst.parser.ParserInterface
 import org.apache.spark.sql.catalyst.plans.logical.LogicalPlan
@@ -162,8 +162,6 @@ private[sql] class SessionState(sparkSession: SparkSession) {
   //  Helper methods, partially leftover from pre-2.0 days
   // ------------------------------------------------------
 
-  def executeSql(sql: String): QueryExecution = executePlan(sqlParser.parsePlan(sql))
-
   def executePlan(plan: LogicalPlan): QueryExecution = new QueryExecution(sparkSession, plan)
 
   def refreshTable(tableName: String): Unit = {

sql/hive/src/test/scala/org/apache/spark/sql/hive/execution/ConcurrentHiveSuite.scala

@@ -30,9 +30,9 @@ class ConcurrentHiveSuite extends SparkFunSuite with BeforeAndAfterAll {
         conf.set("spark.ui.enabled", "false")
         val ts =
           new TestHiveContext(new SparkContext("local", s"TestSQLContext$i", conf))
-        ts.sessionState.executeSql("SHOW TABLES").toRdd.collect()
-        ts.sessionState.executeSql("SELECT * FROM src").toRdd.collect()
-        ts.sessionState.executeSql("SHOW TABLES").toRdd.collect()
+        ts.sparkSession.sql("SHOW TABLES").collect()
+        ts.sparkSession.sql("SELECT * FROM src").collect()
+        ts.sparkSession.sql("SHOW TABLES").collect()
       }
     }
   }