demux_mkv: remove incorrect comment

In the previous commit, I claimed that this GUID stuff was a libavformat
extension, but that seems to be completely wrong. (LATM might be an
extension of some kind, though.)

I don't know what Microsoft calls this GUID "suffix" though. It's
generally used to wrap wav format tags and video FourCCs as GUIDs.
I guess you could grep Microsoft headers for it to find its name,
or something.

demux/codec_tags.c

@@ -75,7 +75,6 @@ static void map_audio_pcm_tag(struct mp_codec_params *c)
         // WAVEFORMATEXTENSIBLE.SubFormat
         unsigned char *subformat = c->extradata + 6;
         if (memcmp(subformat + 4, guid_ffext + 4, 12) == 0) {
-            // libavformat extension.
             c->codec_tag = AV_RL32(subformat);
             c->codec = lookup_tag(c->type, c->codec_tag);
         }