powerpc/pseries/pci: Drop unused MSI code

MSIs should be fully managed by the PCI and IRQ subsystems now.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210701132750.1475580-25-clg@kaod.org