avformat: remove use of deprecated AVFMT_FLAG_KEEP_SIDE_DATA flag

It has no effect whatsoever since the major bump.
Replace the flag's documentation to reflect this as well.

Signed-off-by: James Almer <jamrial@gmail.com>

libavformat/tests/seek.c

@@ -67,8 +67,6 @@ int main(int argc, char **argv)
     int frame_count = 1;
     int duration = 4;
 
-    ic->flags |= AVFMT_FLAG_KEEP_SIDE_DATA;
-
     for(i=2; i<argc; i+=2){
         if       (!strcmp(argv[i], "-seekforw")){
             seekfirst = atoi(argv[i+1]);