ARM: ixp4xx: Drop stale Kconfig entry

ARM: ixp4xx: Drop stale Kconfig entry

The Kconfig entry for the Motorola PrPMC1100 was added to the
kernel in the very first git import for v2.6.12-rc2 in 2005.
But it was never used for anything since it was not accompanied
by any boardfile.

It is easy to support with a device tree if someone needs it,
delete this stray Kconfig.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20220211223238.648934-5-linus.walleij@linaro.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>