configs/roc_pc_rk3399: remove defconfig

This defconfig has been failing to build for several months, with
nobody stepping up to fix it. It's time to drop it.

See
https://lore.kernel.org/buildroot/20220806224338.0159e15c@windsurf/
for a notification on August 6, 2022 about the issues with this
defconfig.

Fixes:

  https://gitlab.com/buildroot.org/buildroot/-/jobs/3259671226



Cc: Suniel Mahesh <sunil@amarulasolutions.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>