configs/zynq_zybo: remove defconfig

This defconfig was added in October 2016 and was never updated since
then. It currently fails to build because U-Boot is too old and
doesn't build with host-openssl in version 1.1.x.

On February 13, 2019, the original submitter was notified, but didn't
reply, so let's remove the defconfig for this platform.

Fixes:

  https://gitlab.com/buildroot.org/buildroot/-/jobs/165766194



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