Remove WebRTC-Pacer-LegacyPacketReferencing flag and most usage

This flag has been default-off since Jul 24th (m77 branch) and apart
from a bug fixed on Aug 5th, there have been no reports of issues, so
let's remove it and start cleaning away the old code path.

Most of the usage within RtpSender/PacingController and their
respective unit tests are removed with this CL, but there will be
several more to follow.

Bug: webrtc:10633
Change-Id: I1986ccf093434ac8fbd8d6db82a0bb44f50b514e
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/149838
Reviewed-by: Sebastian Jansson <srte@webrtc.org>
Commit-Queue: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28930}

modules/pacing/pacing_controller.h

@@ -51,16 +51,6 @@ class PacingController {
                                const PacedPacketInfo& cluster_info) = 0;
     virtual std::vector<std::unique_ptr<RtpPacketToSend>> GeneratePadding(
         DataSize size) = 0;
-
-    // TODO(bugs.webrtc.org/10633): Remove these when old code path is gone.
-    virtual RtpPacketSendResult TimeToSendPacket(
-        uint32_t ssrc,
-        uint16_t sequence_number,
-        int64_t capture_timestamp,
-        bool retransmission,
-        const PacedPacketInfo& packet_info) = 0;
-    virtual DataSize TimeToSendPadding(DataSize size,
-                                       const PacedPacketInfo& pacing_info) = 0;
   };
 
   // Expected max pacer delay. If ExpectedQueueTime() is higher than
@@ -210,11 +200,6 @@ class PacingController {
 
   TimeDelta queue_time_limit;
   bool account_for_audio_;
-
-  // If true, PacedSender should only reference packets as in legacy mode.
-  // If false, PacedSender may have direct ownership of RtpPacketToSend objects.
-  // Defaults to true, will be changed to default false soon.
-  const bool legacy_packet_referencing_;
 };
 }  // namespace webrtc