mm: remove redundant test in validate_page_before_insert()

The page_has_type() call would have included slab since commit
46df8e73 and now we don't even get that far because slab pages have a
zero refcount since commit 9aec2fb0.

Link: https://lkml.kernel.org/r/20250910142923.2465470-3-willy@infradead.org


Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Acked-by: David Hildenbrand <david@redhat.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>