remove unneccecary arguments from mp3_open (#629)

libs/fmt/fmt.c

@@ -519,7 +519,7 @@ HL_PRIM int HL_NAME(mp3_decode_frame)( fmt_mp3 *o, char *bytes, int size, int po
 
 #define _MP3 _ABSTRACT(fmt_mp3)
 
-DEFINE_PRIM(_MP3, mp3_open, _BYTES _I32);
+DEFINE_PRIM(_MP3, mp3_open, _NO_ARG);
 DEFINE_PRIM(_VOID, mp3_frame_info, _MP3 _REF(_I32) _REF(_I32) _REF(_I32) _REF(_I32) _REF(_I32))
 DEFINE_PRIM(_I32, mp3_decode_frame, _MP3 _BYTES _I32 _I32 _BYTES _I32 _I32);