mod: do not attempt to parse standard MIDI files

VLC has a native parser with higher priority. So ModPlug only got to
parse the broken files that VLC rejected. ModPlug is not robust and it
crashes in that case.
(cherry picked from commit 76b15d4038e1f2200ab463c275c0854848de8671)

modules/demux/mod.c

@@ -447,7 +447,6 @@ static int Validate( demux_t *p_demux, const char *psz_ext )
         {  0, "Extended Module" },      /* XM */
         { 44, "SCRM" },                 /* S3M */
         {  0, "IMPM" },                 /* IT */
-        {  0, "MThd" },                 /* MID */
         {  0, "GF1PATCH110" },          /* PAT */
         { 20, "!SCREAM!" },             /* STM */
         { 20, "!Scream!" },             /* STM */