Minor fix

The value "env" is never used in SparkContext.scala.
Add detailed comment for method setDelaySeconds in MetadataCleaner.scala instead of the unsure one.

Author: WangTao <barneystinson@aliyun.com>

Closes apache#1105 from WangTaoTheTonic/master and squashes the following commits:

688358e [WangTao] Minor fix

core/src/main/scala/org/apache/spark/SparkContext.scala

@@ -224,7 +224,6 @@ class SparkContext(config: SparkConf) extends Logging {
 
   /** A default Hadoop Configuration for the Hadoop code (e.g. file systems) that we reuse. */
   val hadoopConfiguration: Configuration = {
-    val env = SparkEnv.get
     val hadoopConf = SparkHadoopUtil.get.newConfiguration()
     // Explicitly check for S3 environment variables
     if (System.getenv("AWS_ACCESS_KEY_ID") != null &&

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

@@ -91,8 +91,13 @@ private[spark] object MetadataCleaner {
     conf.set(MetadataCleanerType.systemProperty(cleanerType),  delay.toString)
   }
 
+  /**
+   * Set the default delay time (in seconds).
+   * @param conf SparkConf instance
+   * @param delay default delay time to set
+   * @param resetAll whether to reset all to default
+   */
   def setDelaySeconds(conf: SparkConf, delay: Int, resetAll: Boolean = true) {
-    // override for all ?
     conf.set("spark.cleaner.ttl", delay.toString)
     if (resetAll) {
       for (cleanerType <- MetadataCleanerType.values) {