Get rid of deprecated version of NackSender::SendNack [1/2]

[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=stefan@webrtc.org

Bug: webrtc:10336
Change-Id: Ia132ef071b1f379fc74834178e75e981ca908125
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/144042
Commit-Queue: Elad Alon <eladalon@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28413}

modules/include/module_common_types.h

@@ -70,14 +70,14 @@ class NackSender {
   // // 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) = 0;
+      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
   // have been added, the feedback message is triggered.
   virtual void SendNack(const std::vector<uint16_t>& sequence_numbers,
-                        bool buffering_allowed) {}
+                        bool buffering_allowed) = 0;
 
  protected:
   virtual ~NackSender() {}