configs/at91sam9g20dfc: remove defconfig, broken

This defconfig no longer builds:

  https://gitlab.com/buildroot.org/buildroot/-/jobs/11012272524

It is still based on the 4.8.6 kernel, which is not even an LTS
kernel, so we cannot easily update to fix the issues which are most
likely due to the newer gcc/binutils versions we're using now.

There is no DEVELOPERS entry attached to this configuration as well,
and this defconfig has been broken for at least 2 months, probably
more (our Gitlab CI history is only kept for ~2 months). So let's drop
it, unless someone volunteers to resurrect it.

Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Cc: Ludovic Desroches <ludovic.desroches@microchip.com>
Cc: Nicolas Ferre <nicolas.ferre@microchip.com>
Cc: Patrice Vilchez <Patrice.Vilchez@microchip.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Arnout Vandecappelle <arnout@rnout.be>
(cherry picked from commit f9e0dc8ddf)
Signed-off-by: Titouan Christophe <titouan.christophe@mind.be>

configs/at91sam9g20dfc_defconfig

@@ -1,21 +0,0 @@
-BR2_arm=y
-BR2_PACKAGE_HOST_LINUX_HEADERS_CUSTOM_4_8=y
-BR2_LINUX_KERNEL=y
-BR2_LINUX_KERNEL_CUSTOM_VERSION=y
-BR2_LINUX_KERNEL_CUSTOM_VERSION_VALUE="4.8.6"
-BR2_LINUX_KERNEL_DEFCONFIG="at91_dt"
-BR2_LINUX_KERNEL_DTS_SUPPORT=y
-BR2_LINUX_KERNEL_INTREE_DTS_NAME="at91sam9g20ek"
-BR2_TARGET_ROOTFS_UBI=y
-BR2_TARGET_ROOTFS_UBIFS_MAXLEBCNT=2047
-BR2_TARGET_AT91BOOTSTRAP3=y
-BR2_TARGET_AT91BOOTSTRAP3_CUSTOM_GIT=y
-BR2_TARGET_AT91BOOTSTRAP3_CUSTOM_REPO_URL="https://github.com/linux4sam/at91bootstrap.git"
-BR2_TARGET_AT91BOOTSTRAP3_CUSTOM_REPO_VERSION="v3.10.3"
-BR2_TARGET_AT91BOOTSTRAP3_DEFCONFIG="at91sam9g20eknf_uboot"
-BR2_TARGET_UBOOT=y
-BR2_TARGET_UBOOT_BUILD_SYSTEM_KCONFIG=y
-BR2_TARGET_UBOOT_CUSTOM_VERSION=y
-BR2_TARGET_UBOOT_CUSTOM_VERSION_VALUE="2016.09.01"
-BR2_TARGET_UBOOT_BOARD_DEFCONFIG="at91sam9g20ek_nandflash"
-BR2_PACKAGE_HOST_SAM_BA=y