mm/gup: remove no longer used gup_fast_undo_dev_pagemap

mm/gup: remove no longer used gup_fast_undo_dev_pagemap

This helper is no longer used after commit fd2825b0760a ("mm/gup: remove
pXX_devmap usage from get_user_pages()").

Link: https://lkml.kernel.org/r/20251219-gup-cleanup-v1-1-348a70d9eecb@tencent.com
Signed-off-by: Kairui Song <kasong@tencent.com>
Acked-by: David Hildenbrand (Red Hat) <david@kernel.org>
Reviewed-by: Alistair Popple <apopple@nvidia.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Peter Xu <peterx@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>