buildroot

configs/ci40: remove defconfig

>From the original submitter of this defconfig:

"""
I no longer have access to ci40 board. It would be difficult for me to
maintain it.
"""

And this defconfig currently fails to build with gcc 6.x. Therefore,
drop it.

Fixes:

  https://gitlab.com/buildroot.org/buildroot/-/jobs/55306806

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