ppc4xx: remove dead code

Since commit 843125da (ppc4xx: remove HH405 board), CONFIG_HH405
is not defined.

Since commit d5263304 (ppc4xx: remove PMC405), CONFIG_PMC405
is not defined.

Acked-by: Stefan Roese <sr@denx.de>
Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Matthias Fuchs <matthias.fuchs@esd.eu>