Cleanup FEC code after refactoring

This CL removes length field from Packet class, as COW buffer data
already has length.

Bug: webrtc:10750
Change-Id: I5c2a857b72007e82e819e7fa5f5aeb2e074730fa
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/144942
Commit-Queue: Ilya Nikolaevskiy <ilnik@webrtc.org>
Reviewed-by: Artem Titov <titovartem@webrtc.org>
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Reviewed-by: Rasmus Brandt <brandtr@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28540}

test/fuzzers/ulpfec_header_reader_fuzzer.cc

@@ -27,7 +27,6 @@ void FuzzOneInput(const uint8_t* data, size_t size) {
       std::min(size, static_cast<size_t>(IP_PACKET_SIZE));
   packet.pkt->data.SetSize(packet_size);
   memcpy(packet.pkt->data.data(), data, packet_size);
-  packet.pkt->length = packet_size;
 
   UlpfecHeaderReader ulpfec_reader;
   ulpfec_reader.ReadFecHeader(&packet);