ffmpeg: remove unused MetadataMap.

This struct was forgotten after
e5df74dc56.

ffmpeg.c

@@ -117,15 +117,6 @@ typedef struct {
     int ofile_idx, ostream_idx;               // output
 } AudioChannelMap;
 
-/**
- * select an input file for an output file
- */
-typedef struct MetadataMap {
-    int  file;      ///< file index
-    char type;      ///< type of metadata to copy -- (g)lobal, (s)tream, (c)hapter or (p)rogram
-    int  index;     ///< stream/chapter/program number
-} MetadataMap;
-
 static const OptionDef options[];
 
 #define MAX_STREAMS 1024    /* arbitrary sanity check value */