powerpc/configs: Drop no longer needed PCI_MSI on powernv

In commit a311e738

 ("powerpc/powernv: Make PCI non-optional") we
made PCI (and therefore PCI_MSI) non-optional on powernv, so it
doesn't need to be in the defconfig anymore.

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