configs/pcengines_apu2: remove defconfig, broken

This defconfig no longer builds:

  https://gitlab.com/buildroot.org/buildroot/-/jobs/11012274139

It is using the unmaintained, not even LTS 5.16 kernel, which no
longer builds with recent toolchains.

It has been broken for at least 2 months:

  https://gitlab.com/buildroot.org/buildroot/-/jobs?name=T%3Dpcengines_apu2&kind=BUILD

Unless a fix is provided, this defconfig will be removed.

Fixes:

  https://gitlab.com/buildroot.org/buildroot/-/jobs/11012274139

Cc: Danilo Bargen <mail@dbrgn.ch>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Arnout Vandecappelle <arnout@rnout.be>

DEVELOPERS

@@ -776,8 +776,6 @@ N:	Daniel Sangue <daniel.sangue@sangue.ch>
 F:	package/libftdi1/
 
 N:	Danilo Bargen <mail@dbrgn.ch>
-F:	board/pcengines/apu2/
-F:	configs/pcengines_apu2_defconfig
 F:	package/tealdeer/
 
 N:	Danomi Manchego <danomimanchego123@gmail.com>