powerpc/85xx: remove the kmp204x_defconfig

It is not maintained and thus obsolete. corenet32_smp_defconfig can be
used as reference for the kmcoge4/kmp204x boards.

Signed-off-by: Valentin Longchamp <valentin.longchamp@keymile.com>
Signed-off-by: Scott Wood <oss@buserror.net>