libxml.gyp: remove clang-on-windows specific option

The /U__STDC_VERSION__ option was necessary when using VS2012 headers,
which didn't include nan(), but we're on 2013 now.

BUG=82385
TBR=cpu

Review URL: https://codereview.chromium.org/443453002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@287415 0039d316-1c4b-4281-b951-d872f2087c98

third_party/libxml/libxml.gyp

@@ -248,16 +248,6 @@
             }, {  # else: OS!="win"
               'product_name': 'xml2',
             }],
-            ['clang==1', {
-              'msvs_settings': {
-                'VCCLCompilerTool': {
-                  'AdditionalOptions': [
-                    # VS2012's standard lib doesn't provide nan().
-                    '/U__STDC_VERSION__',
-                  ],
-                },
-              },
-            }],
           ],
         }],
       ],