powerpc/configs: drop unused BACKLIGHT_GENERIC option

Commit 7ecdea4a ("backlight: generic_bl: Remove this driver as it is
unused") removed geenric_bl driver from the tree, together with
corresponding config option.

Remove BACKLIGHT_GENERIC config item from generic-64bit_defconfig.

Fixes: 7ecdea4a

 ("backlight: generic_bl: Remove this driver as it is unused")
Signed-off-by: Andrey Zhizhikin <andrey.zhizhikin@leica-geosystems.com>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Acked-by: Daniel Thompson <daniel.thompson@linaro.org>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Michael Ellerman <mpe@ellerman.id.au>
Cc: Sam Ravnborg <sam@ravnborg.org>
Link: https://lore.kernel.org/r/20201201222922.3183-6-andrey.zhizhikin@leica-geosystems.com

'
Signed-off-by: Arnd Bergmann <arnd@arndb.de>