configs/pine64_rockpro64: remove defconfig

This defconfig tries to build an ARM Trusted Firmware version that
needs an ARM32 toolchain, which is not available as the platform is an
ARM64 one. The correct solution for this is to have a package in
Buildroot for an ARM32 bare-metal toolchain, but this wasn't done in
time for the 2019.08 release.

In order to not release 2019.08 with a broken defconfig, let's remove
it. It can be re-added later once the ARM32 bare-metal toolchain
problem has been resolved.

Fixes:

  https://gitlab.com/buildroot.org/buildroot/-/jobs/278489367



Cc: Shyam Saini <shyam.saini@amarulasolutions.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>