Delete unused struct BitrateStatistics.
Delete unused struct BitrateStatistics.

Bug: None
Change-Id: I7a2eb979f1245779e0221ea8a5606ba7e97ed0e1
Reviewed-on: https://webrtc-review.googlesource.com/70222
Reviewed-by: Erik Språng <3ab8c90c99d6dd7822e0fad803ab0211dd287ae5@webrtc.org>
Reviewed-by: Benjamin Wright <d97cb5a683a42395ddbd3524ae4315b2c4b93d14@webrtc.org>
Reviewed-by: Karl Wiberg <2c708eb0a58047bf403399f50b3da33ffd2dd444@webrtc.org>
Commit-Queue: Karl Wiberg <2c708eb0a58047bf403399f50b3da33ffd2dd444@webrtc.org>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#22932}
ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc