tools: remove unused gflags module

Its only dependant was closure-linter but that was removed in commit
f9dd34d ("tools: replace closure-linter with eslint").  Remove it.

Fixes: #3217
PR-URL: #3220
Reviewed-By: Trevor Norris <trev.norris@gmail.com>

LICENSE

@@ -237,38 +237,6 @@ The externally maintained libraries used by Node.js are:
     THE SOFTWARE.
   """
 
-- python-gflags is located at tools/gflags.  Its license follows:
-  """
-    # Copyright (c) 2006, Google Inc.
-    # All rights reserved.
-    #
-    # Redistribution and use in source and binary forms, with or without
-    # modification, are permitted provided that the following conditions are
-    # met:
-    #
-    #     * Redistributions of source code must retain the above copyright
-    # notice, this list of conditions and the following disclaimer.
-    #     * Redistributions in binary form must reproduce the above
-    # copyright notice, this list of conditions and the following disclaimer
-    # in the documentation and/or other materials provided with the
-    # distribution.
-    #     * Neither the name of Google Inc. nor the names of its
-    # contributors may be used to endorse or promote products derived from
-    # this software without specific prior written permission.
-    #
-    # THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
-    # "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
-    # LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
-    # A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
-    # OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
-    # SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
-    # LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
-    # DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
-    # THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
-    # (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
-    # OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
-  """
-
 - tools/cpplint.py is a C++ linter. Its license follows:
   """
     # Copyright (c) 2009 Google Inc. All rights reserved.