powerpc/mm: Remove redundant #if case

Remove the check of CONFIG_PPC_SUBPAGE_PROT when deciding if
is_hugepage_only_range() is extern or inline. The extern version is in
slice.c and is built if CONFIG_PPC_MM_SLICES=y.

There was no build break possible because CONFIG_PPC_SUBPAGE_PROT is
only selectable under conditions which also mean CONFIG_PPC_MM_SLICES
will be selected.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>