Remove unused stuff from AudioFrame:

- The interleaved_ field. Never set to anything but 'true'. AudioFrame data appears to always be treated as interleaved.
- The Append() method.
- operator-=().

BUG=

Review URL: https://codereview.webrtc.org/1830713003

Cr-Commit-Position: refs/heads/master@{#12152}

webrtc/modules/audio_coding/neteq/sync_buffer.cc

@@ -79,7 +79,6 @@ void SyncBuffer::GetNextAudioInterleaved(size_t requested_len,
       ReadInterleavedFromIndex(next_index_, samples_to_read, output->data_);
   const size_t samples_read_per_channel = tot_samples_read / Channels();
   next_index_ += samples_read_per_channel;
-  output->interleaved_ = true;
   output->num_channels_ = Channels();
   output->samples_per_channel_ = samples_read_per_channel;
 }