Remove old annotated_steps buildbot scripts and dispatcher

tools/bots/gn_build.py

-#!/usr/bin/env python
-#
-# Copyright (c) 2016, 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.
-
-import os
-import os.path
-import shutil
-import sys
-import subprocess
-
-SCRIPT_DIR = os.path.dirname(sys.argv[0])
-DART_ROOT = os.path.realpath(os.path.join(SCRIPT_DIR, '..', '..'))
-
-def main(argv):
-  os.environ["DART_USE_GYP"] = "1"
-  generate_buildfiles = os.path.join(
-      DART_ROOT, 'tools', 'generate_buildfiles.py')
-  gclient_result = subprocess.call(['python', generate_buildfiles])
-  if gclient_result != 0:
-    return gclient_result
-
-  build_py = os.path.join(DART_ROOT, 'tools', 'build.py')
-  build_result = subprocess.call(['python', build_py] + argv[1:])
-  if build_result != 0:
-    return build_result
-  return 0
-
-if __name__ == '__main__':
-  sys.exit(main(sys.argv))