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}