Remove unused variable from RtpVp9RefFinder

Bug: none
Change-Id: Iaa1f2f8272a7e47f50a3572efb2e0765286c8a0e
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/253843
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Philip Eliasson <philipel@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#36144}

modules/video_coding/rtp_vp9_ref_finder.h

@@ -60,10 +60,6 @@ class RtpVp9RefFinder {
 
   void FlattenFrameIdAndRefs(RtpFrameObject* frame, bool inter_layer_predicted);
 
-  // Save the last picture id in order to detect when there is a gap in frames
-  // that have not yet been fully received.
-  int last_picture_id_ = -1;
-
   // Frames that have been fully received but didn't have all the information
   // needed to determine their references.
   std::deque<std::unique_ptr<RtpFrameObject>> stashed_frames_;