Remove enable_simulcast_stats config flag as not needed anymore

Bug: webrtc:9547
Change-Id: Ie50453aa3496d16bfadfc9fdd3e7e6982278cfba
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/176841
Commit-Queue: Eldar Rello <elrello@microsoft.com>
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31492}

api/peer_connection_interface.h

@@ -666,8 +666,6 @@ class RTC_EXPORT PeerConnectionInterface : public rtc::RefCountInterface {
     // Whether network condition based codec switching is allowed.
     absl::optional<bool> allow_codec_switching;
 
-    bool enable_simulcast_stats = true;
-
     //
     // Don't forget to update operator== if adding something.
     //