demux_mkv: get rid of MS structs

See previous commits. This finally replaces directly reading the file
data into a struct with reading them manually. In theory this is more
portable (no alignment issues and other things). For the most part,
it's nice seeing this gone.

demux/stheader.h

@@ -23,7 +23,7 @@
 
 #include "common/common.h"
 #include "audio/chmap.h"
-#include "ms_hdr.h"
+
 struct MPOpts;
 struct demuxer;