powerpc/pseries/hibernation: remove prepare_late() callback

The pseries hibernate code no longer calls into the original
join/suspend code in kernel/rtas.c, so pseries_prepare_late() and
related code don't accomplish anything now.

Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201207215200.1785968-27-nathanl@linux.ibm.com