Removed sync packet support from NetEq.

I could not find a single place it was used, outside of the unittests
for the sync packet support itself.

Review-Url: https://codereview.webrtc.org/2309303002
Cr-Commit-Position: refs/heads/master@{#14130}

webrtc/modules/audio_coding/neteq/packet_buffer.cc

@@ -273,7 +273,7 @@ size_t PacketBuffer::NumSamplesInBuffer(DecoderDatabase* decoder_database,
     Packet* packet = (*it);
     AudioDecoder* decoder =
         decoder_database->GetDecoder(packet->header.payloadType);
-    if (decoder && !packet->sync_packet) {
+    if (decoder) {
       if (!packet->primary) {
         continue;
       }