powerpc/xive: Drop unmask of MSIs at startup

That was a workaround in the XIVE domain because of the lack of MSI
domain. This is now handled.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210701132750.1475580-7-clg@kaod.org