ARM: ixp4xx: remove dead configs CPU_IXP43X and CPU_IXP46X

Commit 73d04ca5 ("ARM: ixp4xx: Delete Intel reference design
boardfiles") removes the definition of the configs MACH_IXDP465 and
MACH_KIXRP435, but misses to remove the configs CPU_IXP43X and CPU_IXP46X
that depend on those removed configs, and hence are dead now.

Fortunately, ./scripts/checkkconfigsymbols.py warns:

MACH_IXDP465
Referencing files: arch/arm/mach-ixp4xx/Kconfig

MACH_KIXRP435
Referencing files: arch/arm/mach-ixp4xx/Kconfig

Remove the dead configs CPU_IXP43X and CPU_IXP46X.

A further quick grep for the name of those two symbols did not show any
use of the two config symbols; so, there are no further clean-up activities
beyond this config removal needed.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>