[POWERPC] Remove DISCONTIGMEM cruft from page.h

This looks like cruft to me, these functions don't exist AFAICT,
and I can't see that it's possible to even enable DISCONTIGMEM on
powerpc anymore. CC'ing some folks who might know better, based on
the who-touched-it-last principle.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>