Remove all references to GYP

webrtc/build/objc_common.gypi

-# Copyright 2016 The WebRTC project authors. All Rights Reserved.
-#
-# Use of this source code is governed by a BSD-style license
-# that can be found in the LICENSE file in the root of the source
-# tree. An additional intellectual property rights grant can be found
-# in the file PATENTS.  All contributing project authors may
-# be found in the AUTHORS file in the root of the source tree.
-
-# ObjC target common prefix header.
-
-{
-  'variables': {
-    'objc_prefix_file': '../sdk/objc/WebRTC-Prefix.pch',
-  },
-  'xcode_settings': {
-    'CLANG_ENABLE_OBJC_ARC': 'YES',
-    'CLANG_WARN_OBJC_MISSING_PROPERTY_SYNTHESIS': 'YES',
-    'GCC_PREFIX_HEADER': '<(objc_prefix_file)',
-    'GCC_PRECOMPILE_PREFIX_HEADER': 'YES'
-  },
-}