Delete unused/unsupported RetransmissionMode constants

Configurability via SetSelectiveRetransmissions was deleted in
https://webrtc-review.googlesource.com/c/119920.

Delete constants kRetransmitFECPackets and kRetransmitAllPackets,
which are never enabled in production code. Also move the declaration
of RetransmissionMode from rtp_rtcp_defines.h to rtp_sender_video.h,
to reduce visibility to applications.

Bug: None
Change-Id: I70dcf7532aa3415a2449d8d807c500c1f149bf6d
Reviewed-on: https://webrtc-review.googlesource.com/c/120053
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26570}

modules/rtp_rtcp/include/rtp_rtcp_defines.h

@@ -96,19 +96,6 @@ enum KeyFrameRequestMethod { kKeyFrameReqPliRtcp, kKeyFrameReqFirRtcp };
 
 enum RtpRtcpPacketType { kPacketRtp = 0, kPacketKeepAlive = 1 };
 
-// kConditionallyRetransmitHigherLayers allows retransmission of video frames
-// in higher layers if either the last frame in that layer was too far back in
-// time, or if we estimate that a new frame will be available in a lower layer
-// in a shorter time than it would take to request and receive a retransmission.
-enum RetransmissionMode : uint8_t {
-  kRetransmitOff = 0x0,
-  kRetransmitFECPackets = 0x1,
-  kRetransmitBaseLayer = 0x2,
-  kRetransmitHigherLayers = 0x4,
-  kConditionallyRetransmitHigherLayers = 0x8,
-  kRetransmitAllPackets = 0xFF
-};
-
 enum RtxMode {
   kRtxOff = 0x0,
   kRtxRetransmitted = 0x1,     // Only send retransmissions over RTX.