Revert "Only enable conference mode simulcast allocations with flag enabled"

This reverts commit 32ca95145c4636374266f5b5d4d1ac43658bc758.

Reason for revert: Internal test failure

Original change's description:
> Only enable conference mode simulcast allocations with flag enabled
> 
> Non-conference mode simulcast screenshares were mistakenly using the
> conference mode semantics in the simulcast rate allocator, which broke
> spec compliant usage in some situation.
> 
> This behavior should only be used when explicitly using the SDP entry
> "a=x-google-flag:conference" in both offer and answer.
> 
> Bug: webrtc:11310, chromium:1093819
> Change-Id: Ibcba75c88a8405d60467546b33977a782e04e469
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/179081
> Reviewed-by: Harald Alvestrand <hta@webrtc.org>
> Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
> Commit-Queue: Florent Castelli <orphis@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#31828}

TBR=ilnik@webrtc.org,hta@webrtc.org,orphis@webrtc.org

Change-Id: I5ccb6e87594f491ba09fe6b837ee24d63db878ca
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: webrtc:11310
Bug: chromium:1093819
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/180801
Reviewed-by: Florent Castelli <orphis@webrtc.org>
Commit-Queue: Florent Castelli <orphis@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31829}
diff --git a/api/video_codecs/video_encoder_config.cc b/api/video_codecs/video_encoder_config.cc
index 19b0135..6efcbf2 100644
--- a/api/video_codecs/video_encoder_config.cc
+++ b/api/video_codecs/video_encoder_config.cc
@@ -55,8 +55,7 @@
       min_transmit_bitrate_bps(0),
       max_bitrate_bps(0),
       bitrate_priority(1.0),
-      number_of_streams(0),
-      legacy_conference_mode(false) {}
+      number_of_streams(0) {}
 
 VideoEncoderConfig::VideoEncoderConfig(VideoEncoderConfig&&) = default;