revid & stream/flv: removed writing of 'flv header' in the flv encoder

We are now only employing flv encoding for use with rtmp, and rtmp does not like the inclusion of the 'flv header' at the start of a
stream, so this functionality has been removed along with error handling of the invalidFlvTag error from rtmp in the rtmp sender.

revid/senders.go

@@ -400,9 +400,6 @@ func (s *rtmpSender) send() error {
 		return errors.New("no rtmp connection, cannot write")
 	}
 	_, err := s.conn.Write(s.data)
-	if err == rtmp.ErrInvalidFlvTag {
-		return nil
-	}
 	return err
 }