[livy] Thread sessions aren't being used and weren't worthwhile

apps/spark/java/conf/livy-defaults.conf.tmpl

@@ -5,7 +5,7 @@
 # What port to start the server on. Defaults to 8998.
 # livy.server.port = 8998
 
-# What session factory to use. The options are `thread`, `process`, and `yarn`.
+# What session factory to use. The options are `process` and `yarn`.
 # livy.server.session.factory = process
 
 # What spark-submit executable path to use to submit spark applications. Defaults to

apps/spark/java/livy-core/src/main/scala/com/cloudera/hue/livy/LivyConf.scala

@@ -27,7 +27,6 @@ object LivyConf {
   val SPARK_SUBMIT_KEY = "livy.server.spark-submit"
 
   sealed trait SessionKind
-  case class Thread() extends SessionKind
   case class Process() extends SessionKind
   case class Yarn() extends SessionKind
 }
@@ -95,14 +94,13 @@ class LivyConf(loadDefaults: Boolean) {
 
   def sessionKind(): SessionKind = getOption(SESSION_FACTORY_KEY).getOrElse("process") match {
     case "process" => Process()
-    case "thread" => Thread()
     case "yarn" => Yarn()
     case kind => throw new IllegalStateException(f"unknown kind $kind")
   }
 
   /** Return the filesystem root. Defaults to the local filesystem. */
   def filesystemRoot(): String = sessionKind() match {
-    case Process() | Thread() => "file://"
+    case Process() => "file://"
     case Yarn() => "hdfs://"
   }
 }

apps/spark/java/livy-server/src/main/scala/com/cloudera/hue/livy/server/Main.scala

@@ -32,7 +32,6 @@ import org.slf4j.LoggerFactory
 object Main {
 
   val SESSION_KIND = "livy-server.session.kind"
-  val THREAD_SESSION = "thread"
   val PROCESS_SESSION = "process"
   val YARN_SESSION = "yarn"
   lazy val logger = LoggerFactory.getLogger(this.getClass)
@@ -136,8 +135,6 @@ class ScalatraBootstrap extends LifeCycle with Logging {
     info(f"Using $sessionFactoryKind sessions")
 
     val (sessionFactory, batchFactory) = sessionFactoryKind match {
-      case LivyConf.Thread() =>
-        (new InteractiveSessionProcessFactory(livyConf), new BatchSessionProcessFactory(livyConf) )
       case LivyConf.Process() =>
         (new InteractiveSessionProcessFactory(livyConf), new BatchSessionProcessFactory(livyConf))
       case LivyConf.Yarn() =>