Remove RegisterRtcpStatisticsCallback callback functions saving taking an extra lock when calling callbacks. Bug: None Change-Id: Ib4537deffa0ab0abf597228e7c0fab7067614f6a Reviewed-on: https://webrtc-review.googlesource.com/c/111821 Reviewed-by: Oskar Sundbom <ossu@webrtc.org> Reviewed-by: Åsa Persson <asapersson@webrtc.org> Reviewed-by: Niels Moller <nisse@webrtc.org> Commit-Queue: Danil Chapovalov <danilchap@webrtc.org> Cr-Commit-Position: refs/heads/master@{#25779}