Remove unused gyp variable

tools/gyp/configurations_android.gypi

 # Copyright (c) 2012, 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.
 
 # Definitions for building standalone Dart binaries to run on Android.
 # This is mostly excerpted from:
 # http://src.chromium.org/viewvc/chrome/trunk/src/build/common.gypi
 
 {
   'variables': {
-    'dart_io_ssl_builtin_roots': 0,
     'android_ndk_root': '<(PRODUCT_DIR)/../../third_party/android_tools/ndk',
   },  # variables
   'target_defaults': {
     'configurations': {
       # It is very important to get the order of the linker arguments correct.
       # Therefore, we put them all in the architecture specific configurations,
       # even though there are many flags in common, to avoid splitting them
       # between two configurations and possibly accidentally mixing up the
       # order.
       'Dart_Android_Base': {
@@ skipping 284 matching lines @@
             ],
           }],
           ['_toolset=="host"', {
             'ldflags': [ '-pthread' ],
           }],
         ],
       },  # Dart_Android_arm64_Base
     },  # configurations
   },  # target_defaults
 }