fftools/ffmpeg: drop free_input_threads()

Stop demuxer threads in ifile_close() instead. Simplifies the demuxer
API.

fftools/ffmpeg.c

@@ -555,7 +555,6 @@ static void ffmpeg_cleanup(int ret)
     for (i = 0; i < nb_output_files; i++)
         of_close(&output_files[i]);
 
-    free_input_threads();
     for (i = 0; i < nb_input_files; i++)
         ifile_close(&input_files[i]);
 
@@ -3934,7 +3933,6 @@ static int transcode(void)
         /* dump report by using the output first video and audio streams */
         print_report(0, timer_start, cur_time);
     }
-    free_input_threads();
 
     /* at the end of stream, we must flush the decoder buffers */
     for (i = 0; i < nb_input_streams; i++) {
@@ -3987,8 +3985,6 @@ static int transcode(void)
     ret = 0;
 
  fail:
-    free_input_threads();
-
     return ret;
 }