Remove FlexfecConfig and replace with specific struct in VideoSendStream.

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-Original-Commit-Position: refs/heads/master@{#16097}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 3d200bd6ac61437ed8808ed395d425c71c829c4a
11 files changed