[ROCKETMQ-292] Delete system.exit in MQAdminStartup

Author: lindzh <linsony0@163.com>

Closes #169 from lindzh/fix_main_system.exit.

tools/src/main/java/org/apache/rocketmq/tools/command/MQAdminStartup.java

@@ -119,7 +119,6 @@ public class MQAdminStartup {
                             ServerUtil.parseCmdLine("mqadmin " + cmd.commandName(), subargs, cmd.buildCommandlineOptions(options),
                                 new PosixParser());
                         if (null == commandLine) {
-                            System.exit(-1);
                             return;
                         }