mm: remove the unused MIGRATE_PFN_DEVICE flag

No one ever checks this flag, and we could easily get that information
from the page if needed.

Link: https://lore.kernel.org/r/20190814075928.23766-10-hch@lst.de
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Ralph Campbell <rcampbell@nvidia.com>
Reviewed-by: Jason Gunthorpe <jgg@mellanox.com>
Tested-by: Ralph Campbell <rcampbell@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>