include/linux/mmzone.h: remove unused early_pfn_valid()

The early_pfn_valid() macro is defined but it is never used.  Remove it.

Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: David Hildenbrand <david@redhat.com>
Link: https://lkml.kernel.org/r/20200923162915.26935-1-rppt@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>