In mm_init_ppc64() we calculate the location of the "IO hole", but then
no one ever looks at the value. So don't bother.

That's actually all mm_init_ppc64() does, so get rid of it too.
Signed-off-by: NMichael Ellerman <michael@ellerman.id.au>
Signed-off-by: NPaul Mackerras <paulus@samba.org>