Removing some old code which looked like it had to do with NACK handling but in reality did nothing.

webrtc/modules/rtp_rtcp/source/rtp_receiver_impl.cc

 /*
  *  Copyright (c) 2012 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.
  */
 
@@ skipping 50 matching lines @@
       rtp_payload_registry_(rtp_payload_registry),
       rtp_media_receiver_(rtp_media_receiver),
       cb_rtp_feedback_(incoming_messages_callback),
       last_receive_time_(0),
       last_received_payload_length_(0),
       ssrc_(0),
       num_csrcs_(0),
       current_remote_csrc_(),
       last_received_timestamp_(0),
       last_received_frame_time_ms_(-1),
-      last_received_sequence_number_(0),
-      nack_method_(kNackOff) {
+      last_received_sequence_number_(0) {
   assert(incoming_messages_callback);
 
   memset(current_remote_csrc_, 0, sizeof(current_remote_csrc_));
 }
 
 RtpReceiverImpl::~RtpReceiverImpl() {
   for (int i = 0; i < num_csrcs_; ++i) {
     cb_rtp_feedback_->OnIncomingCSRCChanged(current_remote_csrc_[i], false);
   }
 }
@@ skipping 23 matching lines @@
   }
   return result;
 }
 
 int32_t RtpReceiverImpl::DeRegisterReceivePayload(
     const int8_t payload_type) {
   rtc::CritScope lock(&critical_section_rtp_receiver_);
   return rtp_payload_registry_->DeRegisterReceivePayload(payload_type);
 }
 
-NACKMethod RtpReceiverImpl::NACK() const {
-  rtc::CritScope lock(&critical_section_rtp_receiver_);
-  return nack_method_;
-}
-
-// Turn negative acknowledgment requests on/off.
-void RtpReceiverImpl::SetNACKStatus(const NACKMethod method) {
-  rtc::CritScope lock(&critical_section_rtp_receiver_);
-  nack_method_ = method;
-}
-
 uint32_t RtpReceiverImpl::SSRC() const {
   rtc::CritScope lock(&critical_section_rtp_receiver_);
   return ssrc_;
 }
 
 // Get remote CSRC.
 int32_t RtpReceiverImpl::CSRCs(uint32_t array_of_csrcs[kRtpCsrcSize]) const {
   rtc::CritScope lock(&critical_section_rtp_receiver_);
 
   assert(num_csrcs_ <= kRtpCsrcSize);
@@ skipping 331 matching lines @@
     // implementations might have CSRC 0 as a valid value.
     if (num_csrcs_diff > 0) {
       cb_rtp_feedback_->OnIncomingCSRCChanged(0, true);
     } else if (num_csrcs_diff < 0) {
       cb_rtp_feedback_->OnIncomingCSRCChanged(0, false);
     }
   }
 }
 
 }  // namespace webrtc