Revert "Allows FEC generation after pacer step."

This reverts commit 75fd127640bdf1729af6b4a25875e6d01f1570e0.

Reason for revert: Breaks downstream test

Original change's description:
> Allows FEC generation after pacer step.
> 
> Split out from https://webrtc-review.googlesource.com/c/src/+/173708
> This CL enables FEC packets to be generated as media packets are sent,
> rather than generated, i.e. media packets are inserted into the fec
> generator after the pacing stage rather than at packetization time.
> 
> This may have some small impact of performance. FEC packets are
> typically only generated when a new packet with a marker bit is added,
> which means FEC packets protecting a frame will now be sent after all
> of the media packets, rather than (potentially) interleaved with them.
> Therefore this feature is currently behind a flag so we can examine the
> impact. Once we are comfortable with the behavior we'll make it default
> and remove the old code.
> 
> Note that this change does not include the "protect all header
> extensions" part of the original CL - that will be a follow-up.
> 
> Bug: webrtc:11340
> Change-Id: I3fe139c5d53968579b75b91e2612075451ff0f5d
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/177760
> Commit-Queue: Erik Språng <sprang@webrtc.org>
> Reviewed-by: Sebastian Jansson <srte@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#31558}

TBR=sprang@webrtc.org,srte@webrtc.org

Change-Id: Ie714e5f68580cbd57560e086c9dc7292a052de5f
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: webrtc:11340
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/177983
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31559}

modules/pacing/task_queue_paced_sender_unittest.cc

@@ -43,10 +43,6 @@ class MockPacketRouter : public PacketRouter {
               (std::unique_ptr<RtpPacketToSend> packet,
                const PacedPacketInfo& cluster_info),
               (override));
-  MOCK_METHOD(std::vector<std::unique_ptr<RtpPacketToSend>>,
-              FetchFec,
-              (),
-              (override));
   MOCK_METHOD(std::vector<std::unique_ptr<RtpPacketToSend>>,
               GeneratePadding,
               (DataSize target_size),