build_patches: remove 0004-ar71xx-4.9-l2tp-stats.patch

Removing the tunneldigger, this patch is no longer needed.

Signed-off-by: Robert Langhammer <rlanghammer@web.de>
Reviewed-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Reviewed-by: Fabian Bläse <fabian@blaese.de>