powerpc: ppc4xx: remove redundant CONFIG_4xx definition

We do not have to define CONFIG_4xx in board config headers
because it is defined in arch/powerpc/cpu/ppc4xx/config.mk.

include/configs/JSE.h defines "CONFIG_4x", not "CONFIG_4xx".
I believe it is a typo because "CONFIG_4x" is not used at all
in other files.
So, I also deleted "CONFIG_4x" in include/configs/JSE.h.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>