Delete decoder specific buffer_pool_size from webrtc::VideoCodec

VideoDecoder no longer uses this VideoCodec class,
thus this member is unused.

Bug: webrtc:13045
Change-Id: I6e46a563e90f2538bf288995a3837d95c00ba9cb
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/230941
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#34896}

api/video_codecs/video_codec.h

@@ -17,7 +17,6 @@
 #include <string>
 
 #include "absl/strings/string_view.h"
-#include "absl/types/optional.h"
 #include "api/video/video_bitrate_allocation.h"
 #include "api/video/video_codec_type.h"
 #include "api/video_codecs/spatial_layer.h"
@@ -135,12 +134,6 @@ class RTC_EXPORT VideoCodec {
   VideoCodecMode mode;
   bool expect_encode_from_texture;
 
-  // The size of pool which is used to store video frame buffers inside decoder.
-  // If value isn't present some codec-default value will be used.
-  // If value is present and decoder doesn't have buffer pool the
-  // value will be ignored.
-  absl::optional<int> buffer_pool_size;
-
   // Timing frames configuration. There is delay of delay_ms between two
   // consequent timing frames, excluding outliers. Frame is always made a
   // timing frame if it's at least outlier_ratio in percent of "ideal" average