MINOR: Respect the default value of partition argument in SimpleConsumerShell

The `partition` argument is not marked as required, and has a default of `0`, according to the tool's help message. However, if `partition` is not provided the command returns with `Missing required argument "[partition]"`. This patch is to fix the required arguments of the tool by removing `partition` from them.

Author: Vahid Hashemian <vahidhashemian@us.ibm.com>

Reviewers: Ewen Cheslack-Postava <ewen@confluent.io>

Closes #1495 from vahidhashemian/minor/simple_consumer_shell_update_required_args

core/src/main/scala/kafka/tools/SimpleConsumerShell.scala

@@ -101,7 +101,7 @@ object SimpleConsumerShell extends Logging {
       CommandLineUtils.printUsageAndDie(parser, "A low-level tool for fetching data directly from a particular replica.")
 
     val options = parser.parse(args : _*)
-    CommandLineUtils.checkRequiredArgs(parser, options, brokerListOpt, topicOpt, partitionIdOpt)
+    CommandLineUtils.checkRequiredArgs(parser, options, brokerListOpt, topicOpt)
 
     val topic = options.valueOf(topicOpt)
     val partitionId = options.valueOf(partitionIdOpt).intValue()