mm/thp: drop follow_devmap_pmd() default stub

mm/thp: drop follow_devmap_pmd() default stub

follow_devmap_pmd() has already been dropped by the commit fd2825b0760a
("mm/gup: remove pXX_devmap usage from get_user_pages()").  The fallback
stub in the header which is now redundant, can be dropped off as well.

Link: https://lkml.kernel.org/r/20250929104643.1100421-1-anshuman.khandual@arm.com
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
Acked-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Dev Jain <dev.jain@arm.com>
Reviewed-by: Alistair Popple <apopple@nvidia.com>
Reviewed-by: Wei Yang <richard.weiyang@gmail.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>