fftools/ffmpeg: drop OutputFile.sq_encode

It is unused since d119ae2fd82a494d9430ff4d4fc262961a68c598

fftools/ffmpeg.h

@@ -612,8 +612,6 @@ typedef struct OutputFile {
     OutputStream **streams;
     int         nb_streams;
 
-    SyncQueue *sq_encode;
-
     int64_t recording_time;  ///< desired length of the resulting file in microseconds == AV_TIME_BASE units
     int64_t start_time;      ///< start time in microseconds == AV_TIME_BASE units
 

fftools/ffmpeg_mux.c

@@ -845,7 +845,6 @@ void of_free(OutputFile **pof)
         return;
     mux = mux_from_of(of);
 
-    sq_free(&of->sq_encode);
     sq_free(&mux->sq_mux);
 
     for (int i = 0; i < of->nb_streams; i++)