Get rid of deprecated version of NackSender::SendNack [2/2]

[1/2] - Make new version pure-virtual, and deprecated version non-pure.
        This will allow deleting the deprecated version from downstream
        projects.
[2/2] - Remove deprecated version.

TBR=sprang@webrtc.org,stefan@webrtc.org

Bug: webrtc:10336
Change-Id: I3904da12ec471980adfb22f2e61304d42de4ec66
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/144043
Reviewed-by: Elad Alon <eladalon@webrtc.org>
Commit-Queue: Elad Alon <eladalon@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28419}

modules/include/module_common_types.h

@@ -20,7 +20,6 @@
 #include "modules/include/module_common_types_public.h"
 #include "modules/include/module_fec_types.h"
 #include "modules/rtp_rtcp/source/rtp_video_header.h"
-#include "rtc_base/deprecation.h"  // TODO(bugs.webrtc.org/10336): Remove.
 #include "rtc_base/system/rtc_export.h"
 
 namespace webrtc {
@@ -67,11 +66,6 @@ class CallStatsObserver {
 // Interface used by NackModule and JitterBuffer.
 class NackSender {
  public:
-  // // TODO(bugs.webrtc.org/10336): Update downstream and remove this method.
-  // Make the one remaining version of SendNack() pure virtual again.
-  RTC_DEPRECATED virtual void SendNack(
-      const std::vector<uint16_t>& sequence_numbers) {}
-
   // If |buffering_allowed|, other feedback messages (e.g. key frame requests)
   // may be added to the same outgoing feedback message. In that case, it's up
   // to the user of the interface to ensure that when all buffer-able messages