lavf/hls: remove redundancy seekable setting

ffio_init_context with NULL seek callback will setting seekable with 0, so
remove the redundancy seekable setting.
Reviewed-by: Steven Liu <lq@onvideo.cn>
Signed-off-by: Jun Zhao <barryjzhao@tencent.com>

libavformat/hls.c

@@ -1905,7 +1905,6 @@ static int hls_read_header(AVFormatContext *s)
         }
         ffio_init_context(&pls->pb, pls->read_buffer, INITIAL_BUFFER_SIZE, 0, pls,
                           read_data, NULL, NULL);
-        pls->pb.seekable = 0;
         ret = av_probe_input_buffer(&pls->pb, &in_fmt, pls->segments[0]->url,
                                     NULL, 0, 0);
         if (ret < 0) {