mm/migrate: remove obsolete comment about device public

Device public memory never had an in tree consumer and was removed in
commit 25b2995 ("mm: remove MEMORY_DEVICE_PUBLIC support").  Delete
the obsolete comment.

Signed-off-by: Ralph Campbell <rcampbell@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Cc: Jerome Glisse <jglisse@redhat.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Christoph Hellwig <hch@lst.de>
Link: http://lkml.kernel.org/r/20200827190735.12752-2-rcampbell@nvidia.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Author: Ralph Campbell

mm/migrate.c

@@ -381,7 +381,7 @@ static int expected_page_refs(struct address_space *mapping, struct page *page)
 	int expected_count = 1;
 
 	/*
-	 * Device public or private pages have an extra refcount as they are
+	 * Device private pages have an extra refcount as they are
 	 * ZONE_DEVICE pages.
 	 */
 	expected_count += is_device_private_page(page);