Remove third-party cityhash, unused since r8992

gyp/cityhash.gyp

Index: gyp/cityhash.gyp
===================================================================
--- gyp/cityhash.gyp	(revision 9056)
+++ gyp/cityhash.gyp	(working copy)
@@ -1,33 +0,0 @@
-{
-  'variables': {
-    'skia_warnings_as_errors': 0,
-  },
-  'targets': [
-    {
-      'target_name': 'cityhash',
-      'type': 'static_library',
-      'standalone_static_library': 1,
-      'include_dirs': [
-        '../include/config',
-        '../include/core',
-        '../src/utils/cityhash',
-        '../third_party/externals/cityhash/src',
-      ],
-      'sources': [
-        '../third_party/externals/cityhash/src/city.cc',
-      ],
-      'direct_dependent_settings': {
-        'include_dirs': [
-          '../third_party/externals/cityhash/src',
-        ],
-      },
-      'conditions': [
-        [ 'skia_os == "android"', {
-          'cflags!': [
-            '-Wall',
-          ],
-        }],
-      ],
-    },
-  ],
-}