powerpc/pseries/cmm: Rip out memory isolate notifier

The memory isolate notifier was added to allow to offline memory
blocks that contain inflated/"loaned" pages. We can achieve the same
using the balloon compaction framework.

Get rid of the memory isolate notifier. Also, we can get rid of
cmm_mem_going_offline(), as we will never reach that code path now
when we have allocated memory in the balloon (allocated pages are
unmovable and will no longer be special-cased using the memory
isolation notifier).

Leave the memory notifier in place, so we can still back off in case
memory gets offlined.

Signed-off-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20191031142933.10779-7-david@redhat.com