Remove NullReceiveStatistics
rtcp_sender accepts nullptr as indication statistics shouldn't be used,
Other uses of NullReceiveStatistcs were already deleted.

BUG=webrtc:8016

Review-Url: https://codereview.webrtc.org/2988143002
Cr-Original-Commit-Position: refs/heads/master@{#19197}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 8a1d2a315fb44a936cdf6eee646b7067e66ee8ea
3 files changed