Revert "Implement rollback for setRemoteDescription"

This reverts commit 16d4c4d4fbb8644033def1091d2d5c941c1b01fa.

Reason for revert: breaks downstream dependency. (The new enum value kRollback is not handled correctly downstream).

Original change's description:
> Implement rollback for setRemoteDescription
> 
> Bug: chromium:980875
> Change-Id: I4575e9ad1902a20937f9812f49edee2a2441f76d
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/153525
> Commit-Queue: Eldar Rello <elrello@microsoft.com>
> Reviewed-by: Harald Alvestrand <hta@webrtc.org>
> Reviewed-by: Henrik Boström <hbos@webrtc.org>
> Reviewed-by: Steve Anton <steveanton@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#29422}

TBR=steveanton@webrtc.org,mbonadei@webrtc.org,aleloi@webrtc.org,hbos@webrtc.org,aleloi@google.com,hta@webrtc.org,shampson@webrtc.org,elrello@microsoft.com

Change-Id: If76f6b672fdc59b7f00dfc7c150abda16614cd04
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: chromium:980875
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/156304
Reviewed-by: Alex Loiko <aleloi@webrtc.org>
Commit-Queue: Alex Loiko <aleloi@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29427}

api/peer_connection_interface.h

@@ -659,9 +659,6 @@ class RTC_EXPORT PeerConnectionInterface : public rtc::RefCountInterface {
     // logs with TURN server logs. It will not be added if it's an empty string.
     std::string turn_logging_id;
 
-    // Added to be able to control rollout of this feature.
-    bool enable_implicit_rollback = false;
-
     //
     // Don't forget to update operator== if adding something.
     //