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.h

 /*
  *  Copyright (c) 2013 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 30 matching lines @@
                                  const uint32_t rate) override;
 
   int32_t DeRegisterReceivePayload(const int8_t payload_type) override;
 
   bool IncomingRtpPacket(const RTPHeader& rtp_header,
                          const uint8_t* payload,
                          size_t payload_length,
                          PayloadUnion payload_specific,
                          bool in_order) override;
 
-  NACKMethod NACK() const override;
-
-  // Turn negative acknowledgement requests on/off.
-  void SetNACKStatus(const NACKMethod method) override;
-
   // Returns the last received timestamp.
   bool Timestamp(uint32_t* timestamp) const override;
   bool LastReceivedTimeMs(int64_t* receive_time_ms) const override;
 
   uint32_t SSRC() const override;
 
   int32_t CSRCs(uint32_t array_of_csrc[kRtpCsrcSize]) const override;
 
   int32_t Energy(uint8_t array_of_energy[kRtpCsrcSize]) const override;
 
@@ skipping 20 matching lines @@
   size_t last_received_payload_length_;
 
   // SSRCs.
   uint32_t ssrc_;
   uint8_t num_csrcs_;
   uint32_t current_remote_csrc_[kRtpCsrcSize];
 
   uint32_t last_received_timestamp_;
   int64_t last_received_frame_time_ms_;
   uint16_t last_received_sequence_number_;
-
-  NACKMethod nack_method_;
 };
 }  // namespace webrtc
 #endif  // WEBRTC_MODULES_RTP_RTCP_SOURCE_RTP_RECEIVER_IMPL_H_