Clean away WebRTC-Vp8DontDropKeyFrames switch

This feature went to stable with M69. Switch is in M69 and M70 banches.
Since tot is now M71 and we have not seen any issues, let's clean this
up.

Bug: webrtc:9634
Change-Id: I708bab55b0443d0873b09dd5b71cdfad72397a7a
Reviewed-on: https://webrtc-review.googlesource.com/98002
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Commit-Queue: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24581}

modules/video_coding/codecs/vp8/libvpx_vp8_encoder.cc

@@ -27,7 +27,6 @@
 namespace webrtc {
 namespace {
 const char kVp8GfBoostFieldTrial[] = "WebRTC-VP8-GfBoost";
-const char kVp8DontDropKeyframeFieldTrial[] = "WebRTC-Vp8DontDropKeyFrames";
 
 // QP is obtained from VP8-bitstream for HW, so the QP corresponds to the
 // bitstream range of [0, 127] and not the user-level range of [0,63].
@@ -153,8 +152,6 @@ vpx_enc_frame_flags_t LibvpxVp8Encoder::EncodeFlags(
 
 LibvpxVp8Encoder::LibvpxVp8Encoder()
     : use_gf_boost_(webrtc::field_trial::IsEnabled(kVp8GfBoostFieldTrial)),
-      prevent_kf_drop_(
-          !webrtc::field_trial::IsDisabled(kVp8DontDropKeyframeFieldTrial)),
       encoded_complete_callback_(nullptr),
       inited_(false),
       timestamp_(0),
@@ -725,7 +722,7 @@ int LibvpxVp8Encoder::Encode(const VideoFrame& frame,
   for (size_t i = 0; i < encoders_.size(); ++i) {
     tl_configs[i] = temporal_layers_[i]->UpdateLayerConfig(frame.timestamp());
     if (tl_configs[i].drop_frame) {
-      if (send_key_frame && prevent_kf_drop_) {
+      if (send_key_frame) {
         continue;
       }
       // Drop this frame.

modules/video_coding/codecs/vp8/libvpx_vp8_encoder.h

@@ -85,7 +85,6 @@ class LibvpxVp8Encoder : public VP8Encoder {
   uint32_t MaxIntraTarget(uint32_t optimal_buffer_size);
 
   const bool use_gf_boost_;
-  const bool prevent_kf_drop_;
 
   EncodedImageCallback* encoded_complete_callback_;
   VideoCodec codec_;