configs/rock64_defconfig: remove defconfig

configs/rock64_defconfig: remove defconfig

The rock64 defconfig is currently broken [1][2] since a while due to
incompatibility between uboot-2017.09-rockchip-ayufan fork and pylibfdt.
Even with the latest uboot-2017.09-rockchip-ayufan fork version [3],
it doesn't build.

The original submitter tried the uboot upstream rock64-rk3328_defconfig
but the board doesn't boot [4].

In order to not release 2020.05 with a broken defconfig, let's remove
it. It can be re-added later once the uboot issue has been resolved.

[1] 2020.05-rc2: https://gitlab.com/buildroot.org/buildroot/-/jobs/563613273
[2] 2020.02: https://gitlab.com/buildroot.org/buildroot/-/jobs/548596102
[3] https://github.com/ayufan-rock64/linux-u-boot/releases/tag/2017.09-rockchip-ayufan-1065-g95f6152134
[4] http://lists.busybox.net/pipermail/buildroot/2020-May/282164.html

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Michał Łyszczek <michal.lyszczek@bofc.pl>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>