avconv: remove a useless line

Doing anything with that field when we are doing streamcopy (so the
encoder is not opened) should have absolutely no effect.

avconv.c

@@ -1135,7 +1135,6 @@ static void do_streamcopy(InputStream *ist, OutputStream *ost, const AVPacket *p
     }
 
     write_frame(of->ctx, &opkt, ost);
-    ost->st->codec->frame_number++;
 }
 
 int guess_input_channel_layout(InputStream *ist)