Remove all references to GYP

Remove all .gyp and .gypi files.
Remove entries from OWNERS files for *.isolate, *.gyp, *.gypi
Remove unused scripts in webrtc/build.

BUG=webrtc:6323
R=henrika@webrtc.org, phoglund@webrtc.org

Review URL: https://codereview.webrtc.org/2509703002 .

Cr-Commit-Position: refs/heads/master@{#15107}

webrtc/modules/audio_coding/codecs/pcm16b/OWNERS

@@ -1,5 +0,0 @@
-
-# These are for the common case of adding or renaming files. If you're doing
-# structural changes, please get a review from a reviewer in this file.
-per-file *.gyp=*
-per-file *.gypi=*

webrtc/modules/audio_coding/codecs/pcm16b/pcm16b.gypi

@@ -1,29 +0,0 @@
-# Copyright (c) 2011 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.
-
-{
-  'targets': [
-    {
-      'target_name': 'pcm16b',
-      'type': 'static_library',
-      'dependencies': [
-        'audio_encoder_interface',
-        'audio_decoder_interface',
-        'g711',
-      ],
-      'sources': [
-        'audio_decoder_pcm16b.cc',
-        'audio_decoder_pcm16b.h',
-        'audio_encoder_pcm16b.cc',
-        'audio_encoder_pcm16b.h',
-        'pcm16b.c',
-        'pcm16b.h',
-      ],
-    },
-  ], # targets
-}