Remove unnecessary static_cast in rtp_video_stream_receiver2

Bug: None
Change-Id: I8f7424c877e07ee585d46adc81b777577c43d796
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/231697
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Commit-Queue: Evan Shrubsole <eshr@google.com>
Cr-Commit-Position: refs/heads/main@{#34977}
diff --git a/video/rtp_video_stream_receiver2.cc b/video/rtp_video_stream_receiver2.cc
index 309d1588..21414ff 100644
--- a/video/rtp_video_stream_receiver2.cc
+++ b/video/rtp_video_stream_receiver2.cc
@@ -861,8 +861,7 @@
 void RtpVideoStreamReceiver2::OnCompleteFrames(
     RtpFrameReferenceFinder::ReturnVector frames) {
   for (auto& frame : frames) {
-    RtpFrameObject* rtp_frame = static_cast<RtpFrameObject*>(frame.get());
-    last_seq_num_for_pic_id_[rtp_frame->Id()] = rtp_frame->last_seq_num();
+    last_seq_num_for_pic_id_[frame->Id()] = frame->last_seq_num();
 
     last_completed_picture_id_ =
         std::max(last_completed_picture_id_, frame->Id());