KVM: PPC: Book3S HV: Remove redundant mtspr PSPB

This SPR is set to 0 twice when exiting the guest.

Suggested-by: Fabiano Rosas <farosas@linux.ibm.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Daniel Axtens <dja@axtens.net>
Acked-by: Paul Mackerras <paulus@ozlabs.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210412014845.1517916-7-npiggin@gmail.com