avformat: stop exporting ffurl_read_complete, ffurl_seek and ffurl_size

They are not in public headers and not used outside of libavformat.
Reviewed-by: Hendrik Leppkes <h.leppkes@gmail.com>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>

libavformat/libavformat.v

@@ -10,9 +10,6 @@ LIBAVFORMAT_$MAJOR {
                 ffio_set_buf_size;
                 ffurl_close;
                 ffurl_open;
-                ffurl_read_complete;
-                ffurl_seek;
-                ffurl_size;
                 ffurl_write;
                 #those are deprecated, remove on next bump
                 url_feof;