ARM: configs: remove oxnas_v6_defconfig

Due to lack of maintainance and stall of development for a few years now,
and since no new features will ever be added upstream, remove support
for OX820 specific defconfig.

Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Daniel Golle <daniel@makrotopia.org>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>