powerpc/configs: Drop unnecessary CONFIG_NUMA_BALANCING_DEFAULT_ENABLED

In commit 9654f95a ("powerpc: Enable NUMA balancing in
pseries[_le]_defconfig") we added CONFIG_NUMA_BALANCING_DEFAULT_ENABLED
to our defconfigs. But it's already enabled by default, so drop it.

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