Make VideoStreamEncoderInterface::SetFecControllerOverride pure virtual

Clean-up after CL #143962

Bug: webrtc:10769
Change-Id: Ibf574b7b012c40846a88aaf2f1e7a6e2526a25ff
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/143963
Commit-Queue: Elad Alon <eladalon@webrtc.org>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28424}

api/video/video_stream_encoder_interface.h

diff --git a/api/video/video_stream_encoder_interface.h b/api/video/video_stream_encoder_interface.h
index 5ca54e3..32bfc49 100644
--- a/api/video/video_stream_encoder_interface.h
+++ b/api/video/video_stream_encoder_interface.h
@@ -100,10 +100,8 @@
 
   // Set a FecControllerOverride, through which the encoder may override
   // decisions made by FecController.
-  // TODO(bugs.webrtc.org/10769): Update downstream projects and then make this
-  // pure-virtual.
   virtual void SetFecControllerOverride(
-      FecControllerOverride* fec_controller_override) {}
+      FecControllerOverride* fec_controller_override) = 0;
 
   // Creates and configures an encoder with the given |config|. The
   // |max_data_payload_length| is used to support single NAL unit