Remove test that uses an excessive amount of isolates and forces out-of-memory.

tests/standalone/io/many_echo_server_test.dart

-// Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
-// for details. All rights reserved. Use of this source code is governed by a
-// BSD-style license that can be found in the LICENSE file.
-// Stress test isolate generation.
-
-#import("echo_server_test.dart", prefix: "single");
-
-class ManyEchoServerTest {
-  static testMain() {
-    for (int i = 0; i < 5000; i++) {
-      single.EchoServerTest.testMain();
-    }
-  }
-}
-
-main() {
-  ManyEchoServerTest.testMain();
-}