The existence of FlexfecConfig is due to a naive design. Now when it is not used on the receiving side (see https://codereview.webrtc.org/2542413002), it is time to remove it from the sending side as well. BUG=webrtc:5654 Review-Url: https://codereview.webrtc.org/2621573002 Cr-Commit-Position: refs/heads/master@{#16097}