powerpc/configs: Drop no longer needed CONFIG_SMP for pseries/ppc64/powernv

In commit 40e27565 ("powerpc/powernv: Always enable SMP when
building powernv") and 270e2dc9 ("powerpc/pseries: Always enable
SMP when building pseries") we forced CONFIG_SMP on for some configs.
Therefore we don't need to set it in those configs anymore.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>