bugfix: remove unnecessary settings from busybox-1.20.2.config

busybox config doesn't need
	CONFIG_CROSS_COMPILER_PREFIX
	CONFIG_PREFIX
I didn't check it via ${ROOT}/package/pkg_busybox/configs/check.sh

Special Thanks to Saito-san

Reported-by: Takeshi Saito <takeshi.saito.xv@renesas.com>
Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>