configs/riotboard: remove defconfig, broken

This defconfig no longer builds:

  https://gitlab.com/buildroot.org/buildroot/-/jobs/11026769450

It has been broken for at least 2 months:

  https://gitlab.com/buildroot.org/buildroot/-/jobs?name=T%3Driotboard&kind=BUILD

It is using the outdated, not even LTS 4.18.x kernel, which most
likely explains why it no longer builds with modern toolchains.

Unless a fix is provided, this defconfig will be removed.

Fixes:

  https://gitlab.com/buildroot.org/buildroot/-/jobs/11026769450

Cc: Nikolay Dimitrov <nikolay.dimitrov@retrohub.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Arnout Vandecappelle <arnout@rnout.be>
(cherry picked from commit caf055a96c)
Signed-off-by: Titouan Christophe <titouan.christophe@mind.be>

DEVELOPERS

@@ -2508,10 +2508,6 @@ F:	package/ofono/
 N:	Niklas Cassel <niklas.cassel@wdc.com>
 F:	configs/qemu_riscv64_nommu_virt_defconfig
 
-N:	Nikolay Dimitrov <nikolay.dimitrov@retrohub.org>
-F:	board/embest/riotboard/
-F:	configs/riotboard_defconfig
-
 N:	Nimai Mahajan <nimaim@gmail.com>
 F:	package/libucl/