Revert "Guard against NPE if CC is used outside of an application"

This reverts commit e909a42.

Author: Andrew Or

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

@@ -310,9 +310,7 @@ private[spark] object ClosureCleaner extends Logging {
 
   private def ensureSerializable(func: AnyRef) {
     try {
-      if (SparkEnv.get != null) {
-        SparkEnv.get.closureSerializer.newInstance().serialize(func)
-      }
+      SparkEnv.get.closureSerializer.newInstance().serialize(func)
     } catch {
       case ex: Exception => throw new SparkException("Task not serializable", ex)
     }