powerpc: Remove _PAGE_SAO stub for book3e/64

Since commit 634093c5 ("powerpc/mm: enable
ARCH_HAS_VM_GET_PAGE_PROT"), _PAGE_SAO is used only in
arch/powerpc/mm/book3s64/pgtable.c

The _PAGE_SAO stub defined as 0 for book3e/64 can be removed.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/715e644fb3c7d992c0b71f6165ab6cf8c682055a.1655706069.git.christophe.leroy@csgroup.eu