configs/riotboard: remove defconfig

configs/riotboard: remove defconfig

This defconfig fails to build with gcc 6.x, even though we switched to
using gcc 6.x as the default gcc version in May 2017.

This defconfig also hasn't seen any non-mechanical update since
November 2015.

So it's time to get rid of it, in order to remove the following build
failure:

  https://gitlab.com/buildroot.org/buildroot/-/jobs/55306938

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>