Fix default TTL for metadata cleaner

It seems to have been set to 3500 in a previous commit for debugging,
but it should be off by default

core/src/main/scala/org/apache/spark/util/MetadataCleaner.scala

@@ -74,7 +74,7 @@ object MetadataCleanerType extends Enumeration {
 // initialization of StreamingContext. It's okay for users trying to configure stuff themselves.
 object MetadataCleaner {
   def getDelaySeconds(conf: SparkConf) = {
-    conf.getInt("spark.cleaner.ttl", 3500)
+    conf.getInt("spark.cleaner.ttl", -1)
   }
 
   def getDelaySeconds(conf: SparkConf, cleanerType: MetadataCleanerType.MetadataCleanerType): Int =