include/linux/mmzone.h describes ARCH_HAS_HOLES_MEMORYMODEL as
relevant when parts the memmap have been free()d. This would
happen on systems where memory is smaller than a sparsemem-section,
and the extra struct pages are expensive. pfn_valid() on these
systems returns true for the whole sparsemem-section, so an extra
memmap_valid_within() check is needed.

On arm64 we have nomap memory, so always provide pfn_valid() to test
for nomap pages. This means ARCH_HAS_HOLES_MEMORYMODEL's extra checks
are already rolled up into pfn_valid().

Remove it.
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: James Morse <james.morse@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>