Make the config internal.

Author: dongjoon-hyun

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

@@ -311,6 +311,7 @@ object SQLConf {
 
   val THRIFTSERVER_INCREMENTAL_COLLECT =
     SQLConfigBuilder("spark.sql.thriftServer.incrementalCollect")
+      .internal()
       .doc("When true, enable incremental collection for execution in Thrift Server.")
       .booleanConf
       .createWithDefault(false)