rtpproto: Remove a misplaced comment

The fdset is a pollfd array nowadays, and it is already populated
at this point.

Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/rtpproto.c

@@ -331,7 +331,6 @@ static int rtp_read(URLContext *h, uint8_t *buf, int size)
     for(;;) {
         if (ff_check_interrupt(&h->interrupt_callback))
             return AVERROR_EXIT;
-        /* build fdset to listen to RTP and RTCP packets */
         n = poll(p, 2, poll_delay);
         if (n > 0) {
             /* first try RTCP */