message: video_layer_allocation: clean up unused code
  remove unused support for more than four spatial layer descriptions
  of temporal layers
  Cr-Commit-Position: refs/heads/main@{#38646}
diff: @@ -184,16 +184,12 @@ bool RtpVideoLayersAllocationExtension::Write(
  }
  ++write_at;
  - { // Number of temporal layers.
  + { // Number of temporal layers per spatial layer (at most kMaxSpatialIds).
  + static_assert(VideoLayersAllocation::kMaxSpatialIds == 4);
  int bit_offset = 8;
  *write_at = 0;
  for (const auto& layer : allocation.active_spatial_layers) {
  - if (bit_offset == 0) {
  - bit_offset = 6;
  - *++write_at = 0;
  - } else {
  bit_offset -= 2;
  - }
  *write_at |=
  ((layer.target_bitrate_per_temporal_layer.size() - 1) << bit_offset);
  }
  @@ -269,8 +265,9 @@ bool RtpVideoLayersAllocationExtension::Parse(
  return false;
  }
  - // Read number of temporal layers,
  - // Create `allocation->active_spatial_layers` while iterating though it.
  + // Read number of temporal layers per spatial layer (at most kMaxSpatialIds),
  + // create `allocation->active_spatial_layers` while iterating though it.
  + static_assert(VideoLayersAllocation::kMaxSpatialIds == 4);
  int bit_offset = 8;
  for (int stream_idx = 0; stream_idx < num_rtp_streams; ++stream_idx) {
  for (int sid = 0; sid < VideoLayersAllocation::kMaxSpatialIds; ++sid) {
  @@ -278,14 +275,7 @@ bool RtpVideoLayersAllocationExtension::Parse(
  continue;
  }
  - if (bit_offset == 0) {
  - bit_offset = 6;
  - if (++read_at == end) {
  - return false;
  - }
  - } else {
  bit_offset -= 2;
  - }
  int num_temporal_layers = 1 + ((*read_at >> bit_offset) & 0b11);
  allocation->active_spatial_layers.emplace_back();
  auto& layer = allocation->active_spatial_layers.back();
  