rtpsender interface: make pure virtual again

after providing default implementations in Chromium tests

BUG=None

Change-Id: I53bf26b3a99416f4005e7df75b9b86dfbf2489cb
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/273100
Commit-Queue: Philipp Hancke <phancke@microsoft.com>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#37941}

api/rtp_sender_interface.cc

@@ -1,36 +0,0 @@
-/*
- *  Copyright 2018 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.
- */
-
-#include "api/rtp_sender_interface.h"
-
-namespace webrtc {
-
-void RtpSenderInterface::SetFrameEncryptor(
-    rtc::scoped_refptr<FrameEncryptorInterface> frame_encryptor) {}
-
-rtc::scoped_refptr<FrameEncryptorInterface>
-RtpSenderInterface::GetFrameEncryptor() const {
-  return nullptr;
-}
-
-std::vector<RtpEncodingParameters> RtpSenderInterface::init_send_encodings()
-    const {
-  return {};
-}
-
-rtc::scoped_refptr<DtlsTransportInterface> RtpSenderInterface::dtls_transport()
-    const {
-  return nullptr;
-}
-
-void RtpSenderInterface::SetEncoderToPacketizerFrameTransformer(
-    rtc::scoped_refptr<FrameTransformerInterface> frame_transformer) {}
-
-}  // namespace webrtc