demux: remove falsification of ff-index

The `ff-index` is just a index of in the track order as seen by lavf.
However, there are many cases where tracks are not related to lavf, when
it's not used or there are virtual tracks addded like DOVI EL track or
merged image tile grid. Don't conjure `ff-index` out of nothing.

Some scripts may be confused when this index does not much ffmpeg. Even
though it has always stated in docs.

Fixes #18244

demux/demux.c

@@ -1003,9 +1003,6 @@ static void demux_add_sh_stream_locked(struct demux_internal *in,
     if (!sh->codec->codec)
         sh->codec->codec = "";
 
-    if (sh->ff_index < 0)
-        sh->ff_index = sh->index;
-
     MP_TARRAY_APPEND(in, in->streams, in->num_streams, sh);
     mp_assert(in->streams[sh->index] == sh);