remove getThreadLocal(), update docs

Author: davies

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

@@ -43,9 +43,8 @@ import org.apache.spark.util.{AkkaUtils, Utils}
  * :: DeveloperApi ::
  * Holds all the runtime environment objects for a running Spark instance (either master or worker),
  * including the serializer, Akka actor system, block manager, map output tracker, etc. Currently
- * Spark code finds the SparkEnv through a thread-local variable, so each thread that accesses these
- * objects needs to have the right SparkEnv set. You can get the current environment with
- * SparkEnv.get (e.g. after creating a SparkContext) and set it with SparkEnv.set.
+ * Spark code finds the SparkEnv through a global variable, so all the threads can access the same
+ * SparkEnv. It can be accessed by SparkEnv.get (e.g. after creating a SparkContext).
  *
  * NOTE: This is not intended for external use. This is exposed for Shark and may be made private
  *       in a future release.
@@ -129,20 +128,12 @@ object SparkEnv extends Logging {
   }
 
   /**
-   * Returns the ThreadLocal SparkEnv, if non-null. Else returns the SparkEnv
-   * previously set in any thread.
+   * Returns the SparkEnv.
    */
   def get: SparkEnv = {
     env
   }
 
-  /**
-   * Returns the ThreadLocal SparkEnv.
-   */
-  def getThreadLocal: SparkEnv = {
-    env
-  }
-
   private[spark] def create(
       conf: SparkConf,
       executorId: String,