fftools/ffmpeg_mux: make ts_copy_start private to muxing code

It is no longer used outside of ffmpeg_mux*

fftools/ffmpeg.h

@@ -598,11 +598,6 @@ typedef struct OutputStream {
     /* dts of the last packet sent to the muxing queue, in AV_TIME_BASE_Q */
     int64_t last_mux_dts;
 
-    // timestamp from which the streamcopied streams should start,
-    // in AV_TIME_BASE_Q;
-    // everything before it should be discarded
-    int64_t ts_copy_start;
-
     // the timebase of the packets sent to the muxer
     AVRational mux_timebase;
     AVRational enc_timebase;