configs/orangepi_plus: 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.

This defconfig did not even have an entry in the DEVELOPERS file.

Fixes:

  https://gitlab.com/buildroot.org/buildroot/-/jobs/3234499140



Cc: Shyam Saini <shyam.saini@amarulasolutions.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>