Remove RtpVideoHeader::discardable flag.

Calculate it when used instead

Bug: webrtc:11358
Change-Id: Ib79a4ce5e48a1a5244925471c005f96c5ec5dfd0
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/173702
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Reviewed-by: Sam Zackrisson <saza@webrtc.org>
Reviewed-by: Sebastian Jansson <srte@webrtc.org>
Reviewed-by: Philip Eliasson <philipel@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31109}

modules/rtp_rtcp/source/rtp_video_header.h

@@ -53,7 +53,6 @@ struct RTPVideoHeader {
     int temporal_index = 0;
     absl::InlinedVector<DecodeTargetIndication, 10> decode_target_indications;
     absl::InlinedVector<int64_t, 5> dependencies;
-    bool discardable = false;
   };
 
   RTPVideoHeader();