[SPARK-20621][DEPLOY] Delete deprecated config parameter in 'spark-env.sh'

## What changes were proposed in this pull request?

Currently, `spark.executor.instances` is deprecated in `spark-env.sh`, because we suggest config it in `spark-defaults.conf` or other config file. And also this parameter is useless even if you set it in `spark-env.sh`, so remove it in this patch.

## How was this patch tested?

Existing tests.

Please review http://spark.apache.org/contributing.html before opening a pull request.

Author: Xianyang Liu <xianyang.liu@intel.com>

Closes #17881 from ConeyLiu/deprecatedParam.

conf/spark-env.sh.template

@@ -34,7 +34,6 @@
 
 # Options read in YARN client mode
 # - HADOOP_CONF_DIR, to point Spark towards Hadoop configuration files
-# - SPARK_EXECUTOR_INSTANCES, Number of executors to start (Default: 2)
 # - SPARK_EXECUTOR_CORES, Number of cores for the executors (Default: 1).
 # - SPARK_EXECUTOR_MEMORY, Memory per Executor (e.g. 1000M, 2G) (Default: 1G)
 # - SPARK_DRIVER_MEMORY, Memory for Driver (e.g. 1000M, 2G) (Default: 1G)

resource-managers/yarn/src/main/scala/org/apache/spark/deploy/yarn/YarnSparkHadoopUtil.scala

@@ -280,10 +280,7 @@ object YarnSparkHadoopUtil {
 
       initialNumExecutors
     } else {
-      val targetNumExecutors =
-        sys.env.get("SPARK_EXECUTOR_INSTANCES").map(_.toInt).getOrElse(numExecutors)
-      // System property can override environment variable.
-      conf.get(EXECUTOR_INSTANCES).getOrElse(targetNumExecutors)
+      conf.get(EXECUTOR_INSTANCES).getOrElse(numExecutors)
     }
   }
 }