configs/ts7680: remove defconfig, broken

This defconfig no longer builds:

  https://gitlab.com/buildroot.org/buildroot/-/jobs/11026774180

It has been broken for at least 2 months:

  https://gitlab.com/buildroot.org/buildroot/-/jobs?name=T%3Dts7680&kind=BUILD

It uses the outdated Linux 4.9 kernel, which is no longer maintained,
and no longer builds with recent toolchains.

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

Fixes:

  https://gitlab.com/buildroot.org/buildroot/-/jobs/11026774180

Cc: Julien Grossholtz <julien.grossholtz@openest.io>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Arnout Vandecappelle <arnout@rnout.be>

DEVELOPERS

@@ -1796,9 +1796,7 @@ F:	package/qt5/
 
 N:	Julien Grossholtz <julien.grossholtz@openest.io>
 F:	board/raspberrypi/
-F:	board/technologic/ts7680/
 F:	configs/raspberrypizero2w_defconfig
-F:	configs/ts7680_defconfig
 F:	package/paho-mqtt-c
 
 N:	Julien Olivain <ju.o@free.fr>